That was the same argument the Southern states made in Loving v. Virginia, the SCOTUS case which struck down the bans on interracial marrigae: everyone has the right to marry one unmarried person of the same race as themselves provided he/she is over a certain age. SCOTUS was not persuaded that this was not discrimination.
Before you flame me, I am not saying SCOTUS is going to overturn Proposition 8, nor do I think it should. But your argument is not a very persuasive one.
Not quite. There is a fundamental distinction between a law based on the fundamental difference of sex, and one based on race. See Baker v. Nelson, 409 U.S. 810 (1972).