Like it or not, think it's pejorative or not, the current logic people are using to justify and legalize gay relationships applies equally to other non-abusive relationships, including polygamy, polyamory and bestiality. Are we saying that homosexuality and beastiality are the same, morally speaking? No, but I'm not sure why bestialists are judged as 'bad' – I mean, it may seem 'gross' to us, but who are we to judge? You can't have it both ways if you want to be logical.
And The People's Cube has a really witty article on this – but if you can't take the humor, and are deeply offended, your misplaced sense of self-righteousness will certainly keep you from seeing the point. Here's the intro: