Opposition to same-sex "marriage" need not have anything to do with visceral disgust for certain acts such as buggery or man-on-man fellatio. I am all too aware that many who oppose same-sex "marriage" do so because they think SS"M" necessarily entails societal approval of homosexual acts. But these people are just wrong.
To include gays in the institution of marriage, it must be re-defined in such a way as to sever all links between it and any particular sexual act. For if we say marriage is about the responsible use of coitus, then we necessarily exclude same-sex couples because they just cannot perform coitus. If we say marriage is about scissoring, well, that excludes male same-sex and heterosexual couples. And so on.
Since the number of sexual acts are innumerable in this age of the Subjective Fetish, a public definition of marriage that aims to comprehend all sexual orientations simply cannot include or even imply any sexual act at all. Otherwise, it might be guilty of irrational, demeaning, and degrading discrimination. And, therefore, the only way to be fair to every single kink out there is to make the public definition of marriage, ironically enough, into nothing more than companionship.
In other words, public marriage becomes asexual, and all sexual activity becomes exactly like religion (should be) in a secular liberal society, a wholly private matter with no public purpose whatsoever. But this is insane. Coitus, unlike any other sexual act, does have a public purpose for without it, we would simply have no public.