I'm not equating homosexuality to bestiality other than homosexuality was seen as a sexual perversion until fairly recently. As a society "advances", things will be more liberated and things that were sexual perversions in the past soon become the new normal. Not saying its going to happen, but I would bet that pedophillia and bestiality are much more prevalent than you would think. Just look at all the arrests on Dateline NBC, teacher scandals, child porn arrests, etc. If society were to tolerate it say 20 years from now, I believe the behavior would likely become more "accepted" and the same arguments will remain in place by a certain type of person.
And when you refer to the "portion of society that is giving poor reasoning and fear mongering", I think the anti-gay marriage crowd is pretty well covered in that regard. Most of them are batshit insane religious types.