IIRC, his first and last name were visible to me in 23 and me, so I googled him and saw that we was a therapist with a focus on the LGBT community, so then I messaged him through 23 and me, told him a bit of my story and asked him if he was gay.

This is not the sort of thing a genetic test can determine

Give it time...

It might not happen. Fingerprints are partly heritable – immigration delay disease, for example, has an identifiable point mutation – but you'd be laughed out of the room if you tried to suggest that given time, we would eventually be able to predict fingerprints from genomes.

Non-heterosexual behaviour can be observed in most social amniotes, even those without fingerprints. Genetically-identical twins do not have identical sexual proclivities. There's no a priori reason to believe there's even a "gay gene" to find.

I looked it up, and twin studies say sexual orientation is party genetic, but it's far from the only factor.

So at most a perfect genetic test could give a percentage chance of someone being gay/straight.

I guess that's pretty much what you said.

And either gayness is evolutionarily beneficial (kin selection?), or it's so fundamental to amniote psychology that evolution can't get rid of it. (Or both, I guess.)

So you're saying they aren't born that way but it's learned? DSM2 called and something something something.

That is very much not what is being said.