My tenth-baked theory is that homosexuality possibly results from issues that the mind can have adjusting to the complexities that human civilization (I’m on US English today) has added to everything, including sexual mores.
Whereas every other species seems happy to do it in the field as it were, humans have hugely complicated the entire matter — arguably also culminating in all the gender spaghetti ideas of today.
For some people it seems that homosexuality might be a way out of problems that the modern expectations of heterosexuality pose for them. The same social pressures possibly also underlie some aspects of feminism’s rise, as well as the MRA and MGTOW movements. Even going a long way back into history, humans were terribly ‘civilized’ when compared to other species.
Ironically, you can sometimes see this theoretical idea evidenced between males and females, if the females know that the males are gay. The ‘threat’ that the heterosexual male seems to pose in some females’ eyes seems to disappear. Women often seem to be overly friendly in a very relaxed way with a male they know is gay.
Anyways — just a wacky hunch I’m throwing out there to be demolished by whoever…