If I understand it correctly, the argument that same sex marriage will harm opposite sex marriage is as follows.
Marriage laws have two purposes. The first is symbolic, i.e., they intend to convey a message about how life should be lived. I don't think proponents of gay marriage can readily deny that since much of the push for the recognition of same-sex marriage is also about the validation of such relationships.
The argument continues, however, to claim that the message that it intends to convey is not concerned with encouraging all relationships of mutual affection and support. Marriage is not open to a variety of such relationships that are not sexually intimate, such as adult children who live with a parent or two siblings who live together. What is it about a sexually intimate relationship that requires recognition by the state and an associated web of rules and responsibilities?
The answer, the argument continues, has to do with procreation. Heterosexual relationships can make babies. They are, in fact, given enough time, likely to do so. This is true whether or not the parties intend to procreate. The failure to use contraception and contraceptive failure is not insignificant. If men and women have sex, babies are a likely outcome.
The state has an interest in doing what it reasonably can to ensure that these children are raised in the best possible environment. A presumption underlying marriage is that children are best off when both parents stick around to raise them and do so in the same household. In other things, all things being equal, they are best off with their mother and father, Thus the state encourages the channeling of heterosexual relationships into marriage to, at least in part, create a social norm that men and women who are in an intimate relationships ought to get married.
The particular benefits and burdens that are incidents of marriage flow from that. In the same-sex marriage debate, it is not uncommon to hear talk of marriage's benefits, but the argument in favor of heterosexual marriage rejects the notion that marriage is about getting access to benefits. Rather, because it involves a relationship that can lead to the creation of children, it imposes upon couples who enter into it a set of reciprocal claims of each on the other, that are all really based on assumptions about what sustains a relationship in which children may be raised.
Thus ,the relationship cannot be freely ended. While we have liberalized the rules surrounding divorce, ending marriage cannot be done without legal sanction and without procedures and consequences that are designed to at least minimize the risk that it may be ended casually. Marriage gives each party a claim on the property and income of the other because we know that childrearing usually results in a financial disabling of one of them, usually the mother. Marriage comes with a norm of sexual exclusivity (until recently enforced by legal sanction) because we know that "open" male-female relationships are inherently unstable. It is limited to two persons because children have one mother and one father.
Before going on, I want to suggest a few responses to customary objections. First is the fact that we do not limit marriage to heterosexuals who wish to or can procreate. In other words, the scope of marriage is overly broad.
I can think of a few answers. The first is that it is not always obvious who those couples are. The second is that the function of law as enforcing a social norm may be served by extending that norm to all heterosexual couples, i.e., by reinforcing the idea that this is what men and women do.
The second objection rests on the fact that we do recognize other kinds of relationships in which children are raised. We no longer stigmatize illegitimacy and many states permit gay couples to adopt. In an ironic twist, the Massachusetts Supreme Judicial Court in Goodridge (the case that found opposite sex-only marriage unconstitutional)held these liberalizing steps against the Commonwealth. If the state permits these things, it reasoned, it cannot possible believe that it is preferable for children to be raised by their own mothers and fathers in the context of marriage.
I think the Goodridge court failed to acknowledge that there is a difference between encouraging the ideal and accepting the actual. We can think it best, in the great run of cases, that children be raised by their own mothers and fathers and still recognize that they will not always be. In light of that, we can conclude that stigmatizing those children whose out-of-wedlock birth is unavoidable is both cruel and harmful. We can acknowledge that, for children who cannot be raised by their own parents, adoption by a same-sex couple may be a good alternative.
Third, it seems to me to be besides the point that some children may fare better with, say, an adoptive same sex couple than abusive natural parents. Social policy is, of necessity, concerned with promoting what is best in the great run of cases.
Marriage is an institution designed to ensure that a child's natural parents stick around to raise her because, in the overwhelming majority of cases, that is what will be best for her. That, thank God, other arrangements might work for those children who cannot enjoy the ideal, says nothing about whether that ideal ought to be encouraged.
In a case in Indiana (Morrison) in which the Supreme Court of that state held that opposite-sex marriage was not unconstitutional, the court more or less stopped here. It concluded that there was a rationale for marriage that doesn't apply to same sex couples so it is not a denial of equal protection to decline to extend marriage to same sex couples. I think the court was right.
But we can't stop there because we are not just concerned with whether the state can limit marriage to homosexuals, but whether it should. If we buy into the argument, so far, all we have so far established is that marriage is to be encouraged and the reasons for encouraging it may not apply to same sex couples (with one caveat I'll get to later). We haven't yet considered how same sex marriage might undercut that rationale.
Update: Go to Shark and Shepherd for parts 3 and 4.