Proponents of gay "marriage" (for the sake of my poor fingers, let the sarcastic quotation marks be merely implied from henceforth) often accuse their opponents of homophobia, a kind of pop-psychological diagnosis that requires no degree to confer, which serves to delegitimize any argument against gay marriage by asserting that it stems from nothing but irrational disgust.
Setting aside, for a moment, the unfairness of the characterization, is disgust irrational?
Disgust appears to be a pretty universal feature of the human condition. I can only assume this is because it serves a valid end. All humans, regardless of race or culture or other peculiarities, developed the ability to communicate verbally precisely because it conferred an unequivocal survival advantage. Likewise with any other relatively universal trait.
So what advantage does disgust toward homosexuality confer? For one thing, homoesxual behavior (at least [or especially] male homosexual behavior) is a disease vector. The peculiarly predatory sexual behaviors common to modern gay men aggravate this; a not insubstantial number of practicing homosexuals (about three-quarters according to some sources) report having had over 100 partners. This is simply not a lifestyle conducive to good health -- in fact, it's conducive to widespread and early death, exactly like we're seeing today. Disgust innoculates communities against it, leading them to avoid, shun, and exile (literally or figuratively) homosexual men in their midst.
So the etiology of man's disgust toward homosexuals is completely rational even if you grant the left's materialist/biodeterminist presuppositions. Why, then, does modern man hate it?
Because disgust, like heterosexuality, arises spontaneously from human nature -- and modern man detests the natural order of being and wishes to destroy it.
So he will embrace literally anything that is contrary to it (decriminalize it, then legalize it, then demand that it be publicly tolerated, then demand that it be publicly endorsed, then demand that it be subsidized), and oppose literally anything that arises from it.
Because human nature itself is "homophobic," the (true) rightist wants homosexuality criminalized or at least stigmatized -- while the leftist would rather keep homosexuality and abolish human nature, instead.