In many of the ethical quandaries of our time, most case involve having to give up something. In the case of animal welfare, for example, those arguing for animal freedom are asking others to give up eating meat and using animal products. In the case of climate change, we are being asked to cut back, or even give up, on certain greenhouse-emitting products and activities.
The case of gay marriage is odd in that respect, as there is really nothing that anyone has to give up. If there is an inequality, it seems quite trivial that it be rectified. Of course, it doesn't really stop reasons to that effect being proposed. The most common one is the institution of marriage itself - that marriage itself is what loses out. Or more precisely, the notion of the family unit.
While it seems irrelevant to the discussion, a lot is made of reproductive capabilities. Marriage, as the argument follows, is an institution to support procreation. Homosexuals since they can't procreate, therefore should have no rights when it comes to marriage. While straight childless couples can still marry, it doesn't matter because the practice is to support the procreation and raising of children.
The problem with this line of argument is that even if that is the reason to have marriage as an institution, in no way is that capacity diminished by gay marriage. Married heterosexual couples still have exactly the same rights as before, only that homosexual couples are able to have the other benefits that stem from marriage.
I tend to think that the problems with gay marriage aren't so much about reproductive rights, but that it would be affording homosexuality equal societal recognition. The real loss, in this case, I suspect is the perception of superiority of a particular kind of relationship. it's a threat to the ego in much the same way as giving women or aboriginals the right to vote, and realistically has as much justification for failing to grant such a right.