It's not the same argument at all.

Why not beastiality? Consent.

Why not polygamy? Agency, inherent power inequality.

The reason why gay marriage won't "degenerate to polygamy and beastiality" isn't because people are somehow reasonable and know where to draw the line on the slippery slope.

It's because there's no slippery slope at all.

We have involuntary registration of various violent offenders for several decades now. Some offenders like sexual offenders actually have to go and make it known to all of their neighbors that they are a sexual offender. However I have yet to see it degenerate to people having to go around admitting that they voted for Bush. Slippery slopes are only as slipper as people want then to be. Society knows very well where to draw the line usually if it didn't it wouldn't function, you might not like where it draws it but it's not up to you to decide that on your own anyhow.

The constitution -- and the fundamental inalienable rights it enumerates -- were written to provide a backstop for when society fails to draw the line on a slippery slope.

This is exactly what happened with the legalization of gay marriage through the judicial branch.

Your comment is incredibly naive in the face of even contemporary world history, nevermind the generations of slave and feudalistic societies that came before us.