Although I'm not well-versed in genetics, if this were true, wouldn't it be a paradox? Given that homosexual persons usually don't procreate, the gene wouldn't be passed on! When I think about it that way, I can't understand how the theory even came into existence.

I believe homosexuality is psychological, just like any sexual preference. Say for instance a straight man has a fetish for women with hairy armpits. He's not born loving women with hairy armpits, but due to various psychological factors during his development, it ends up being something he's attracted to, and there's really nothing he can do about it.