Rick Hills has a series of posts on gay marriage and the slippery slope argument. Eric Posner responds:
The slippery slope argument has become shorthand for the view that if we try to make sure that all our social and legal conventions are morally justified, we will end up with no conventions or (what is almost the same thing) in a state of permanent transition between different conventions, which would be worse than tolerating existing conventions that are unjust. This argument is, at the extreme, an objection to any time of reform, and so cannot always carry the day, but it is not an argument that one can refute simply by showing that the proposed new convention is morally superior to the old one. But the reason this argument persists in the same-sex-marriage debate is that it is impossible to point out any concrete harms from recognizing same-sex marriage, and thus the only argument left is this anxiety about the arbitrariness of conventions.
The Dish 