This may take a while. Bear with me.
One of the difficulties in arguing for the amendment is the need to constantly restate what you are not saying. My arguments for the amendment have nothing to do with homosexuality about which I make no moral judgment. It is not about whether gay and lesbian relationships should exist or legal. They do exist and they are legal. The question is not even whether the state should recognize or facilitate certain legal arrangements that may be useful to some (but based on the low rate of same sex marriage in Massachusetts, not most) of these relationships. The issue is - to the virtual exclusion of all else - preserving further erosion of the privileged status of marriage and the tailoring of the social norms and rules surrounding it to the nature of the relationship for which it was designed.
This does not mean that I think that individual people "who love each other" are themselves a threat to marriage or that, in a world of same-sex marriage, straight people will "turn gay" (although I do believe that it is within the prerogative of society to, while not ostracizing or mistreating homosexuals, treat heterosexuality as normative).
What I worry about is that, if marriage is seen as something that whose purpose is merely to facilitate or sanctify loving relationships or that its purposes must bend to some blunt notion of equality, two broad trends are bolstered - one that is already well underway and the other relatively new but inextricably linked to the push to redefine marriage.
First, if marriage is largely about love, i.e., love not only a necessary but sufficient condition for marriage, then we reinforce the idea that a marriage ought to exist only so long as the parties think they love one another. Note that this is not an argument that gays and lesbians are more likely to break up. It is solely based on further reinforcement of the notion that marriage is a private matter that ought to be whatever the individual parties to it want it to be. We have traveled quite a way down that road already with ugly results. I prefer to stop.
Second, the rules and social norms around marriage largely flow not just from "love" but from the nature of heterosexual relationships, i.e., from the fact that men and women experience sexuality differently and, therefore, often have competing interests in the relationship and that these relationships, if they last long enough, will probably produce children - often unintentionally. All of the "rules" of marriage, i.e., monogamy, sexual exclusivity (related to but distinct from the first), presumption of paternity, presumption of financial interdependence, etc., are, to a greater or lesser degree, rooted in these distinctives.
These ideas may work for some same-sex relationships. In debates, the no side almost always produced a lesbian in a long term relationship with a child that was adopted or the product of artificial insemination. (Although most children in such relationships are, in fact, the product of earlier heterosexual relationships, my sense is that they stayed away from couples like that because it raises a whole set of difficult questions.) For these couples, maybe something like marriage works well.
They are, however, a small minority of same sex couples. What works for the others? If it is not the same as marriage, do we run the risk of eroding the norms and rules around marriage in the interests of inclusivity and equality? Do we run the risk of creating further public confusion around what marriage entails?
The other risk of the redefinition of marriage is that we have no real basis to deny marital "benefits" and "protections" to other relationships that are loving and mutually dependent. Some may be polygamous or polyamorous (again, this is not to say that gays or lesbians are particularly drawn to that), but some may be non-conjugal.
One of the reasons that I worry about this as a possibility is that there are advocates of same sex marriage who argue that this is precisely where it will - and should - lead. Stanley Kurtz provides some examples but there are more. There is a whole legal literature about "deprivileging marriage" and "marital equality" is thought to be a step in that direction.
This does not mean that all of the advocates of same sex marriage are closet polygamists, communards or that they all want to end marriage. It does mean that some very smart (albeit very radical) people have seen the implications of the movement.
This brings us, at long last, to Exhibit C. It is Fair Wisconsin's signature ad which is, perhaps, one of the most cynically brilliant and intellectually dishonest spots I have ever seen. Karl Rove and Lee Atwater have nothing on these guys.
It is one of the most surreal things I have ever seen. People who sincerely believe that the unavailability of marriage or its equivalent to homosexuals is unjust are arguing that we should vote "no" because the injustice will continue. The not so subtle association of the word "no" with "gay marriage" is clearly designed to confuse voters as to what a "yes" and "no" vote means. It is the functional equivalent of telling voters in traditionally Democratic precincts that, due to long lines at the polls, Democrats will be voting on Wednesday.
Why can't Fair Wisconsin have the courage of its convictions? Is it really because the general public is made up of homophobes who can't be trusted to do the "right" thing? The degree to which society accepts gays and lesbians has increased exponentially. That can't be right.
I'd suggest that it is because the public intrinsically knows that changing what may be our most fundamental institution in a way that, until a few years ago, no one in any place and at any time would have dreamt of is likely to change it in ways that we can't predict.