Bestiality is a really bad example to counter an argument for gay marriage. Sure on the most basic level, you might be keeping that person who wants to fuck horses or whatever from being happy, but realistically, no one is ever gonna want to marry a horse. Why? They have nothing to gain legally by making a horse their partner. If they go to prison, the horse will not willingly go to conjugal visits. If he dies and the horse gets everything, what is the horse gonna do with it? Nothing. Not to mention I don't think I've seen it even once where someone wanted to marry an animal, compared to the number of gay couples that have wanted to/want to get married.
Give them the same rights I say, but don't call it marriage so other people don't get all offended that the institution of marriage is getting violated. If gay people have a problem with it not being called marriage, they can fuck off.