If people equate two adults marrying to an adult/child, or human/animal marriage you have a few screws loose in your head. That's like making the argument that if a grown man and a grown woman decide to get married, then what's stopping a grown man from marrying an underage girl? I mean, the logic is exactly the same as two grown men or two grown women getting married = grown man and underage boy or grown woman and underage girl getting married.
There's no slippery slope because we have laws against pedophilia/statutory rape and bestiality. You're saying that a grown human male is the same thing as a male dog if you think that gay marriage will lead to inter-species marriage.
This goes for "grandfather/nephew marriages." Wtf?? lol. Seriously? Again, we have incest laws in place. At least Canada does... I know some states down south allow some incest between cousins or some shit. And those states scream the loudest again gay marriage LOL.