Marriage is an institution that doesn't discriminate. If Mr A is hetero and Mr B is homo, neither can marry Mr C, no matter his proclivities. To claim that discriminates against Mr. B is ludicrous on it's face.
What's more, any legislation limiting the newspeak meaning of marriage to homosexuals would be discriminatory.
So gay "Mr. B" can't marry gay "Mr. C", but it is OK for him to marry gay "Ms. D"? A lot of homosexuals are in marriages of convenience. They do so to get the benefits that a heterosexual person can or to hide their proclivities from others. I find that more repugnant than two homosexuals in a committed relationship. Shouldn't society promote stable, loving families? Doesn't that benefit us all?