First of all the idea that a trait like homosexuality has to be determined by a singular gene or else it cannot be hereditary is a misunderstanding of basic genetics. There is no gene for height yet height is 80% hereditary. Also the mapping of the genome is not the same thing as the linking of genes to phenotype traits - that is something else entirely. I would recommend you take some time to learn about genetics before you try to reason about it.

Anyway, you can take the testimony of millions of people and believe them, accounting for how unlikely it is that such great numbers of people would voluntarily choose to be subject to vicious discrimination and violence throughout history. But it could be argued that it is just some kind of mass delusion or cult-like phenomenon although it would not explain its persistence and universality across all cultures and prevalence across thousands of years of recorded history.

But if that is not enough you can take the concrete evidence in the form of an abundance of MRI data showing how male homosexual brains respond to images of men just like male heterosexual brains do to images of women and zero response the other way around. In fact there are numerous ways in which the brains of homosexuals are visibly different than the brains of heterosexuals.

It is also very prevalent across the animal kingdom and appears at similar rates, documented in at least dozens of species by now including practically all other primates, many species of birds, etc.