So every gay character needs to have some kinda gay origin story? I think if you asked a member of the gay community why they were gay they would it's because they fancy people of the same sex. Their whole family wasn't butchered by someone of the oposite gender, they didn't fall into a vat tank of gay serum and they weren't mind controlled into only wanting sex where there is no chance of reproduction by an evil genius with a very long view into the destruction of mankind.
Sometimes people are gay, it doesn't require a backstory.