Legally gay?

Who knows who is gay and who isn't? And does it matter?

Yes, it matters. It seems that the Supreme Court and others think it's worse to pick on gays than it is to do the same to non-gays, in the name of "equal protection under the law".

Unfortunately they didn't tell us how to determine if someone is gay. This problem doesn't exist to the same degree with other groups SCOTUS has seen fit to protect. You can't necessarily tell if someone is black or a woman or handicapped 100% of the time either, but at least there are credentials like a birth certificate or driver's license. (Handicaps are ripe for scams this way too, as noted here, but that is not the issue du jour).

So what would happen if a landlord or other would-be discriminator asked the potential victim if they were gay or not? I've seen some ungodly questionnaires for tenant applicants and I don't recall ever seeing such a request. Would this be unlawful?

But if the landlord can't be shown to have known if the victim was gay or otherwise, how can they be shown to have discriminated? Surely this takes intent.

No, I'm not a lawyer, and it shows. But IMO if the law is going to take homosexuality into account for special protections, then people should be permanently identified as being gay or straight to prevent abuse of the system.