Oh ok, I missed that part.
Anyways I'm willing to bet that polygamy and incestual marriages are coming. There has already been many cases in Ontario and BC of Muslim polygamous couples claiming the constitutional right to marry, using the same arguments gay marriage advocates have used. I think we'll legalize polygamy in Canada within the next decade. Incest will also follow.
If you say that this is a slippery slope fallacy, well I say that it is not because the slippery slope is only a fallacy when it is not backed by a rational argument. In this case, if you claim, as gay marriage advocates do, that the only criteria for marriage is love rather than the complementarity of the sexes which used to be the rule (a man and a woman), then yes, then not legalizing polygamy and incest is discriminatory.