Eugene Volokh posts about same-sex marriage and slippery slopes.
Yeah, yeah, a slippery slope is a metaphor and not an argument. But the previous statement is not an argument either - invocation of the slippery slope metaphor does not invalidate an argument.
In this case IMO everything boils down to one question:
What legal, logical or moral argument(s) would a supporter of gay marriage use to oppose extending marriage beyond permitting same-sex couples? If these are unsound or nonexistent, then in fact we have a slippery slope - if we can't limit marriage to a pair consisting of a man and a woman, can we limit it at all?
Don't expect me to trust people not to push beyond a certain point. They will, if only out of orneriness.
Or for tax benefits.