For some people, marriage is still an institution that is defined by proscribed roles. The man has certain rights and responsibilities (mostly the former), and the woman has certain rights and responsibilities (mostly the latter), and these are handed down by God and should not be questioned — and even if you don’t agree with the theological angle, our culture has defined it that way, so you’ll be safe from Jeebus’ fig tree-hatin’ wrath either way.
Same-sex marriage, by not filling the “appropriate” genders, challenges the notion that proscribed gender roles are necessary for a successful marriage. If two men can have an effective relationship, and one of them fulfills the role that was “supposed” to be assigned to the woman (or, GASP, even more sinful, if they work out their own individual division of responsibilities in an equitable and loving way, that doesn’t necessarily conform to 1950s gender stereotypes — oh god I can’t believe I typed that GET THEE BEHIND ME SATAN!) then the next thing you know, women in heterosexual relationships will be wavering on the whole “unquestioned obedience” principle. It’s a slippery slope, you know?
Despite some sarcasm in that last paragraph, I’m not joking at all. Marriage equality poses a direct threat to the patriarchy. So in that sense, the wingnuts are dead-on accurate: If your definition of the institution of marriage inherently requires a patriarchal arrangement, marriage equality is corrosive towards that institution.*applauds*
Bravo. I have nothing to add to that.