heretohelp,
It depends on how you use the “slipperly slope” argument. Arguing because we allowed A we will allow B is unconvincing.
Arguing that just as you agree we can prohibit B, we can also prohibit A is a convincing argument.
If, as the Supreme Court held, moral disapproval is not a valid basis for prohibiting something, then it is very hard to justify why bestiality, poygamy and incest (where both parties are adults) can be prohibited. The other reasons given for prohibiting those acts, like those regarding mishkav zachor, do not hold up to an serious scrutiny.