From the Family Research Council
What’s wrong with letting same-sex couples legally “marry?”
There are two key reasons why the legal rights, benefits, and responsibilities of civil marriage should not be extended to same-sex couples.
The first is that homosexual relationships are not marriage. That is, they simply do not fit the minimum necessary condition for a marriage to exist–namely, the union of a man and a woman.
The second is that homosexual relationships are harmful. Not only do they not provide the same benefits to society as heterosexual marriages, but their consequences are far more negative than positive.
Either argument, standing alone, is sufficient to reject the claim that same-sex unions should be granted the legal status of marriage.
Let’s look at the first argument. Isn’t marriage whatever the law says it is?
No. Marriage is not a creation of the law. Marriage is a fundamental human institution that predates the law and the Constitution. At its heart, it is an anthropological and sociological reality, not a legal one. Laws relating to marriage merely recognize and regulate an institution that already exists.
But isn’t marriage just a way of recognizing people who love each other and want to spend their lives together?
If love and companionship were sufficient to define marriage, then there would be no reason to deny “marriage” to unions of a child and an adult, or an adult child and his or her aging parent, or to roommates who have no sexual relationship, or to groups rather than couples. Love and companionship are usually considered integral to marriage in our culture, but they are not sufficient to define it as an institution.