Prior to the Charter of Rights, the definition of marriage was rooted in human biology (male-female). It was open to all, regardless of sexual orientation, and in that sense was completely non-discriminatory. The Charter judges, against the wisdom of Parliament, added "sexual orientation" as a new basis for rights-based thinking.
The problem is, there are many sexual orientations, and the courts to this point have chosen to grant only one sexual identity group marriage rights based on their identity. We know that bisexuals are a separate class of persons, because homosexual advocacy groups tell us so; yet the courts have not recognized "bisexual marriage", i.e., the rights of a bisexual who is attracted to both a man and a woman to marry them both.
We know that pederasty is a common sexual orientation -- perhaps as common as homosexuality, and no doubt equally "unchosen". If all sexual orientations are "equal" under the Charter, then why don't we have pederast marriage? And, if they are not all equal, well, then maybe homosexuality, as a behaviour, really isn't equal, either. It's a slippery slope, once you deny the equality of all sexual orientations.
And what about "swingers"? -- we know that they are an identifiable "minority", because the courts have said so. Would those who claim to believe in the Charter deny swingers'-marriage, involving multiple partners? (What, a one night stand is splendid, commitment is immoral?) If not, we are right back to the two-classes of Canadians argument, which according to our present Prime Minister, "we simply cannot have".
I do not believe in specific rights and entitlements based on "sexual orientation". But if I did, I would at least insist that all sexual orientations be treated as equal, and not just one. The fact that proponents of same-sex equality have left the battle, and are not going to bat for pederasts, swingers, and bisexuals is troublesome. If even supporters don't actually believe in it, why should we?