I think we agree. Slippery slope arguments are valid if 1) there is no demarcated line ("extingent circumstances") or 2) there is a demarcated line, and there

I think we agree. Slippery slope arguments are valid if 1) there is no demarcated line ("extingent circumstances") or 2) there is a demarcated line, and there is good justification for why that line might change. Legalizing gay marriage is a clearly demarcated line. There is no justification for why gay marriage being legalized will lead to bestiality being legalized, so it is an invalid slippery slope argument.