Yes, the good old slippery slope argument. It never fails to get folks riled up. Especially folks that support gay marriage. They say such an argument is nonsense. Which would be the case if it weren't for the middle step.
See, before they get to the bit about bestiality/pedophilia/objectophilia/etc., they usually invoke the dreaded polyamory! *cue chipmunk again* Gay marriage leads to polyamory leads to literally anything goes. That middle step makes their argument half right. After all, if gender isn't a restriction on marriage, why should number be?
Of course, the key thing here is informed consent. Steps one and two involve contracts between consenting adults; step three does not. Children, animals, and objects can't give legal consent, thus they can't get married. So, there's no need to worry. Allowing people of the same gender, or three or more people, to marry won't result in people suddenly marrying goats and dishwashers, and the like.
Mind you, what I just did there requires logic, and most folks can't seem to get past their different = evil mentality long enough to use it. *sigh*