As I understand anti-prejudice concepts, We The People have decreed that we should not discriminate against folks who have natural differences. A Black man didn't choose to be black. A child with birth defects didn't make that choice. Nor other handicapped. No job/housing/schooling discrimination based on male/female genes.
Discrimination is legal, if based on choice, NOT genes. You choose to rob banks, you become a felon and lose rights. You choose to molest children, your life turns to crap.
So, until there is a proven scientific cause for homosexuality, I don't see how the choice of sexual partners counts towards discrimination, or the lack there of. (Hmm, if homosexuality were once more considered a disease, ADA might help out?)
Of course, the simpler way around the question of gay marriage might be to abolish any legal distinction for marriage at all. No "married filing jointly", no automatic inheritance. Parenthood decided by genes except by contract. Prostitution legalized. Marriage would be a personal contract.
Contract "til death do you part", or for a chosen term? Hmm, with prenups as they are, "Term Marriages" are probably legal now, aren't they?
Of course all that I said is subject to the laws passed by We The People. Not many people seem to want to change the gay marriage situation. Or the hetero rules either.