This article helps to illustrate why:
BOSTON (AP) — Massachusetts, the first state to legalize gay marriage, sued the U.S. government Wednesday over a federal law that defines marriage as a union between a man and a woman.
The federal Defense of Marriage Act interferes with the right of Massachusetts to define and regulate marriage as it sees fit, Massachusetts Attorney General Martha Coakley said. The 1996 law denies federal recognition of gay marriage and gives states the right to refuse to recognize same-sex marriages performed in other states.
The lawsuit, filed in federal court in Boston, argues the act "constitutes an overreaching and discriminatory federal law." It says the approximately 16,000 same-sex couples who have married in Massachusetts since the state began performing gay marriages in 2004 are being unfairly denied federal benefits given to heterosexual couples.
"They are entitled to equal treatment under the laws regardless of whether they are gay or straight," Coakley said at a news conference to discuss the lawsuit. [...]
The Massachusetts lawsuit challenges the section of the federal law that creates a federal definition of marriage as "a legal union between one man and one woman as husband and wife."
Gay marriage advocates, you see, aren't content with getting a few states who really don't care much about marriage and are perfectly willing to subsidize immorality, cohabitation, serial marriage, out-of-wedlock childbirth, and a host of other social deviations from the marriage norm to call their unions a sort of "marriage." They want to force all of us to do so--and in doing so, they want to interfere with my marriage--and yours. How?
By redefining what marriage is.
It's true that marriage has, over the centuries of human existence, had a few major variations. Those variations ended up resolving into two competing definitions: marriage was one man and one woman, or marriage was one man and more than one woman. There really haven't been, in the past few thousand years, any major variations other than these, and since the dawn of the Christian world polygamy slowly began to die out. Certainly by the time our Constitution was written it was taken for granted that the word "marriage" implied a contract, religious, legal, or both, between one man and one woman.
Why have such a contractual relationship? Most relationships are either relationships of blood or relationships of chosen association. A cousin may be a friend, but a friend doesn't have to be a cousin; blood ties and chosen ones may sometimes overlap, but the two belong to different categories of relationship.
If marriage is, as gay marriage advocates insist that it is, a relationship of chosen association, then why have any sort of contract around it? One doesn't enter into a legal relationship every time one forms a new friendship, or joins a social club (most of them, anyway) or otherwise ventures into a voluntary chosen association with members of his community. Why should a man and a woman, or a man and a man, or a woman and a woman demand a contractual bond around their relationship when the relationship itself is nothing more than a kind of friendship, one which frequently, but not necessarily, involves some sort of sexual activity? Plenty of people engage in that activity with other friends, or even causal strangers, to whom they aren't married and have no desire to marry. So what makes this thing we call "marriage" different enough, or special enough, or important enough to require a contract?
The reality is that marriage is the one type of relationship which is designed to be both a relationship of chosen association and a relationship of blood. While it is true, as the gay marriage apologists insist on every opportunity, that not every marriage between a man and a woman will produce children, the nature of the relationship is such that the possibility that each marriage may produce children must be considered, especially in the contracts dealing with such relationships. A tiny handful (relatively speaking) of heterosexual marriages will fail to produce children, either because of infertility or advanced age; but the presence of these few infertile heterosexual marriages does not change the biological reality that the vast majority of heterosexual marriages will produce children--that is, that the relationship, which begins as a relationship of chosen association, will become a relationship of blood.
In fact, the traditional understanding of marriage considered the "blood relationship" part of the marriage relationship to begin, not with the birth of the first child, but with the marriage act itself. Once a couple had consummated the marriage a divorce or annulment used to be much harder to get (in the civil law sense, of course; Catholic annulments are a different matter, as the validity of the marriage, not its consummation, was and is the vital point). The fact that the couple had entered into marriage and then consummated the marriage with the act necessary to reproduction meant that theirs was no longer a mere relationship of chosen association--they were understood to be "one flesh," and were, in many ways, treated by the law as one person. Whether or not they were then, in due time, parents of children who truly were related to them both, equally, by blood was a secondary (though still important) consideration.
If we look at this understanding of civil marriage and compare it to the reality of gay relationships, we can see that there is a fundamental difference between gay relationships and heterosexual marriage. A marriage, once consummated, creates a relationship of both chosen association and of blood between the man and the woman who have entered it; the blood relationship may merely be in anticipation of their potential parenthood, but it is still a reality. A gay relationship cannot honestly be said ever to be "consummated" in the sense in which that word is ordinarily used; the specific type of physical encounter which has always been an important element of a valid marriage is not even possible for two men or two women, to state, as delicately as possible, the blindingly obvious. Moreover, there is no possibility for any gay couple of potential parenthood as a couple. While they can manufacture children using IVF or other immoral means, raise children from previous heterosexual encounters, or adopt children, they can never create as a couple the relationship of blood--not just a few gay couples because of accidental or age-related infertility, but all gay couples; no gay couple can ever become parents of their own biological children as a couple.
So by insisting that the word marriage must be redefined in such a way that two men or two women can "have one" so to speak, gay marriage advocates are interfering with your marriage and mine. Their view of marriage is that it is only a relationship of chosen association, like friendship, club membership, or other (usually temporary) voluntary associations. A gay marriage can never be a blood relationship, because a gay couple can't even potentially become the parents of their own biological children as a couple. The assumptions which go along with blood relationships completely fail when gay relationships take place: the mother of a son, for example, doesn't expect an additional son to be added to the family by marriage, but a daughter; grandparents don't expect to have to "share" their granddaughter with her two dads, her biological mom, and the biological mom's parents, in addition to the second "dad's" parents who have no biological connection whatsoever with the child their son is raising, but whom the child is taught to call "Grandma" and "Grandpa," giving her a total set of three such grandparents.
So gay marriage doesn't just redefine marriage--it redefines family, blood relationships, and these ties for generations at a time. Gay marriage doesn't just interfere with marriage; ultimately, it will impact all of civilization. And I think it will quite likely destroy it.