Supporters of legalized gay marriage insist that there is no threat to marriage and, hence, that the arguments of opponents are ill-founded. Even if one concedes the truth of at least some threat on the margins of marriage, that isn’t enough to justify opposing gay marriage because bans on gay marriage devalue same-sex couples and hold gay citizens in a second-class status. You can't protect a social institution through bigotry, injustice, and discrimination.
The same is true when it comes to arguing that legalizing gay marriage might lead to legalizing polygamy or incestuous marriages. Discrimination against gays is no more a justified means for protecting marriage than discrimination against blacks, atheists, or women would be. Anyone who tried to argue that some minority should enjoy reduced civil rights in order “protect marriage on the margins” would be justifiably labeled as bigoted against the minority.
It is reasonable to hold the same position with respect to opponents of gay marriage as well: by arguing that we should discriminate against gay citizens as a whole in order to protect some marriages “on the margins,” they are just as bigoted as someone arguing that we should discriminate against black citizens or atheists citizens in order to protect marriage (or any other cultural institution) on the margins. Protection of marriage is a laudable goal, but discrimination isn’t a justified means.
It's no coincidence that people who want to "protect marriage" never volunteer to suffer a disadvantage; instead, they are always insisting that others must make the involuntary sacrifice. It's always some minority that is the threat for expecting equal treatment. If it could be proven that conservative religious beliefs are bad for marriage, do you suppose conservative Christians would agree to be shut out of marriage and relegated to a "separate but equal" status like "Religious Union"? I doubt it..