The historic structure of human society is rooted in natural law and common law. Natural law flows from nature itself. It is existentially true, and confirmed by natural science, that all of Nature operates by and is governed according to a lawful order.
Natural law is the human understanding of the intrinsic Laws of Nature. To explain, "natural law" is the phrase for man's understanding of the "Law of Nature." For such things as gravity, which is obvious to everyone, there is really no difference between the two phrases. For some points of natural science, however, our understanding of what is truly in Nature might be deficient, and in that case to use the phrase natural law is more appealing and more cautious than to claim that we truly know what the Law of Nature is.
Over thousands of years of human history, the recognition of elements of natural law has settled into the legal arena in Western society as English common law. Components of common law such as inalienable rights of life, liberty and property underlie the positive laws issued by legislative bodies and the case laws arising from judges' decisions.
Christians (and probably all other theists) hold that nature is the creation of God, and thus that natural law is rooted in the authority of God as Creator. True Christians have also held the Bible to be the written Word of God, with affirming historical evidence, and therefore Christians hold that the Bible is a reliable source for moral and judicial principles and practices. Coherent interpretation of both authoritative sources, nature and Scripture, is thus a reliable basis for law and policy. Historians recognize that Western society is largely developed on this basis. Christian historians in particular have accepted that God has spoken with authority in both nature and Scripture. Because God is not the author of confusion, correct interpretation of both nature and Scripture logically can be harmonious, leading to coherence in law and policy.
Concerning marriage, the traditional view that marriage is a union between male and female is affirmed by nature and its particular component, human biology. Marriage as the union of male and female is based on the obvious evidence of human anatomy and sexual propagation of the human race. The physiology and functions of male and female are complementary. Nature teaches sexual bonding and nurture of the young. As this complementarity is fundamental in human society from its beginnings, marriage is to be regarded as a Creation ordinance, given by the Creator as a foundational fact of human existence. All human society is subject to this Creation ordinance, no matter what views are held over the entire range of thought and practice.
Marriage is seen by some commentators on Scripture as instituted by God in the Garden of Eden. Therefore they say it was not birthed by the Church (which came later), but was instituted at the beginning of mankind made in the image of God. It pre-existed the Church and the Bible. Thus marriage is seen as a gifting in nature itself, that is, marriage is from the Law of Nature. Therefore it is binding on all human beings, not just those who are saved. From a theological perspective, marriage is a Creation ordinance. This is essentially the same thing as saying that marriage is from the Law of Nature.
The institution of an ecclesiastical ceremony of marriage, for Jews and Christians, developed long after creation of the human race. Ceremonies honoring marriage exist even in pagan cultures.
While some respond that the natural law argument for traditional marriage has been set aside by many theologians, it does not lose its force by mere opinion. Marriage as a Creation ordinance is applicable to all human beings. In both pagan societies as well as in those with religious roots such as Christian and Jewish, male and female have bonded together in what we call marriage -- permanent bonding together. Lifetime bonding is also found in pairings of various animals.
The traditional view of marriage is also declared by Scripture in both Old and New Testaments, and especially implied by the words of Jesus when He referred to the making of male and female. Traditional marriage in Scripture is seen as a reflection of the relation between God and His Church, the latter called the Bride of Christ. Scripture clearly rejects homosexual behavior as an affront to God, and inconsistent with His purpose and intentions for marriage and family life. Homosexual acts are termed an “abomination” in the Old Testament. Modern interpretations of Scripture that supposedly weaken its support for traditional marriage, and allow homosexual practice, are beyond the pale of historic theology grounded in early Church practice and endorsed by over 2000 years of Christian history. No positive endorsement of homosexual activity can be found in Scripture, only its proscription.
The consequence is that an endorsement of homosexual behavior is a direct attack on the authority of Scripture.
Asserting that Scripture does not apply to the pagan because he does not believe it (and thus he is free to neglect it) is contrary to Scripture as God’s Word. Scripture sees itself as applying to all human beings. Certainly, God’s law is uniform, fixed, universal, and eternal.
Thus, God has spoken in both Scripture and Creation with a coherent message – marriage is between only a man and a woman, and homosexual behavior is disordered and sinful. Consequently, one cannot legitimately call it marriage when there is some sort of legal union between those of the same sex similar to marriage, despite contrary popular sentiment. The redefinition of the term marriage currently espoused by advocates of “same-sex marriage” is a classic political tactic for altering traditional mores.
One of the points that might arise in discussion about homosexual marriage is that Roman Catholics see marriage as a sacrament, while the Westminster Confession and Protestants in general do not. The Roman Catholic position on sacraments, if held to be true in a discussion, will be a stronger barrier to “homosexual marriage” than the Protestant view on sacraments.
Some advocates of homosexual practice argue that the early Church approved of slavery and of subjugation of women, and as both approvals were later rejected, so the present day Church resistance to homosexual activity should also be reversed. However, claiming an early Church approval of slavery and subjugation of women is not sustained by a careful reading of history and of Scripture. Thus there is insufficient parallel between those issues and proscription of homosexual activity to justify abandoning the latter.
Summing up to this point, positive laws now being enacted establishing homosexual commitments as marriage cannot validly overturn thousands of years of human history -- such laws ignore the evidence of human anatomy, undermine the common law, and contravene natural law. For the Christian, positive laws establishing homosexual commitments as marriage also disregard God’s word in Scripture, and institutionalize sin.
There are features of human relationships that should be kept separate from the “same-sex marriage" question. Such features include the right of inheritance, visiting rights in hospitals, groupings for health insurance policies, and ordinary commercial transactions involving the elements necessary for human existence (e.g., food and shelter), and so on. All such features concerning basic human rights that are separable from marriage can be protected by ordinary social contracts (via civil law), and in many situations are already protected.
What we should oppose is having public policy transform the legal definition of marriage so that it no longer protects traditional marriage and children. Children if possible should grow up in a traditional family structure, having a father and a mother. Current changes in society concerning marriage no longer give advantage and preference to traditional family structure, and the consequence is a further erosion of social stability. A positive feedback loop has been established whereby such erosion will accelerate.
Protecting traditional marriage is not only consistent with Scripture and nature (especially human biology), it is sound public policy. Nature, as evidenced in abundant social science research, demonstrates that children psychologically benefit the most when raised in traditional family structures. Traditional families are further blessed economically because stable traditional families are optimal in avoiding resource waste, preserving wealth, and maintaining a beneficial social and ethical environment.
Endorsing so-called "civil unions" as an alternative offered to homosexual couples, giving them all the benefits of marriage, is really a linguistic sleight-of-hand that should be resisted. Implementing “homosexual marriage” is not a case of extending equality to homosexuals, because “homosexual marriage” is not the equivalent of traditional marriage, and can never be equivalent, because of obvious differences between the sexes.
Some claim that “homosexual marriage” is a “civil right”. Valid civil rights are based on true human rights, and these do not include redefining marriage or using "civil union" as its equivalent just with another name. A true human right is based in nature (such as inalienable rights), and is not created simply by a legislative decision at variance with nature.
When an event or function in society is not related to the question of traditional marriage, there should be no discrimination against homosexuals. However, when the event or function of interest would involve the direct endorsement of a novel view of marriage (such as marrying homosexual partners or having homosexual wedding receptions, etc.), the owners of the venue have the inalienable right to, and should be permitted to, refuse the function, as a matter of law and conscience, based on nature and Scripture.
It is not only a protection of freedom of religion that is involved, but also an inalienable right of existence of all individuals, to be able to resist affirming a practice contrary to both natural law and conscience. Society should not allow legislated law to overturn inalienable rights and religious freedom.
We should note that proponents of “homosexual marriage” will have no basis in natural law for denying polygamy, group marriage, bestiality, pederasty, and everything else that is worse including theft, murder and assassination, because such proponents have no foundation left in natural law, having abandoned it at the outset when they assert that “homosexual marriage” is valid. Their only recourse is to assert that a group of human beings should be allowed to do whatever the group decides. This is equivalent to asserting that man himself is the maker of all law. Such a view is contrary to the natural order established in Creation and in human beings in particular, as affirmed both by physical evidence and by revealed religion, that man is not the maker of himself nor is he the maker of Laws of Nature.
When Christians or pagans cast away the natural law argument, two consequences will ensue –
1) Christians will have no persuasive argument about traditional marriage for people who are pagan and who reject the Bible and the Church. Christians need the natural law argument to persuade pagans concerning traditional marriage.
2) Pagans will have no persuasive argument about why any moral or ethical principle should rule, such as a prohibition of murder or theft, or bestiality, or pederasty, because they will not be able to root their view in nature with any consistency, to support their view. Consequently, they will have to simply say that a group of people can make up their own minds about ethics and morals and civil laws. They will have no basis for saying their morals are the correct ones, or better than another group's. They will have become a law unto themselves -- making up their own laws. That leaves children and families open to confusion and destruction. Evidence for such confusion is already abundant in current society.
To sum up, marriage is not simply ecclesiastical (arising primarily from the Church and originating in Scripture), but originates in our nature as human beings where male and female anatomy and physiology and function are complementary. A traditional marriage is optimal for the spouses and their children, including society at large. Scripture affirms homosexual behavior as sinful and deserving of judgment.
How then should a church respond to homosexuals? The full gospel reveals God’s love for sinners. God’s love as revealed in Scripture and history is rooted in His righteousness, the fixity of His laws, and His provision in Christ for redemption of sinners. His judgments are unavoidable, yet they are superintended by His mercy. “Mercy does triumph over judgment [James 2:13], but not by simply setting it aside; rather, mercy triumphs over judgment because judgment is fully executed and justice finally satisfied at the cross.” An individual’s salvation depends on his or her acceptance of God’s law and mercy brought together at the cross. Thus an attitude of welcome and inclusion in a church, originating in a desire by the community to be loving and merciful, should not be promoted to the neglect of God’s law.
Modern research has disclosed that a small percentage of human beings have an inclination toward homosexual attraction. There is no consensus from research whether this inclination is inherent, or is a result of influences during human development. A much larger percentage have an inclination toward heterosexual promiscuity. Both inclinations need to be resisted.
All unsaved sinners are welcome as visitors in a church, where they can receive by grace from the Spirit of Christ Who is Present, and be encouraged to confess Christ as Savior and Lord, leading to repentance from sin. However, the church should not condone any sin, except temporarily to facilitate the working out of “salvation with fear and trembling" (Philippians 2:12). Because “it is the glory of God to conceal a matter” (Proverbs 25:2), quiet efforts at therapy for confessing sinners are appropriate.
For certain, the church must not endorse sin, as in the case of implementing “marriage” ceremonies for homosexual couples, or receiving homosexual couples as members. Criteria for all church staffing should be maintained according to Scriptural prescriptions. Practicing homosexuals who persist in attending a local church without changing their behavior may have to be rebuked in the manner treated in 1 Corinthians 5:1f. It will be disruptive to the assembly, and an affront to God in Scripture, if a culture is promoted within the church to accept homosexual behavior as normative rather than sinful.
 With respect to physical laws, such as gravity, it is obvious that no one can escape their rule. It is not so obvious that moral rules and ethics also cannot be avoided, but it is easily discerned that breaking various moral rules in society has negative consequences, even if they arise only over a long period of time, rather than instantly, as in the case of physical laws such as gravity.
 From the Hebrew tow' ebah -- morally disgusting, but with implication of idolatry.
 Nicholas Batzig, "Mercy Triumphs Through Judgment," http://www.ligonier.org/learn/articles/mercy-triumphs-through-judgment/.
God Ordained Family Unit