i am tempted to say i think homosexuality is caused by hot people of the same sex, but i think you wanted a serious answer
so far the most convincing evidence i’ve seen is that it’s primarily about hormone and chemical levels in the womb during pregnancy, combined with certain amount environmental surroundings and genetics. that wouldn’t explain everything - for instance, while twins are more likely to both be gay than other sets of siblings, there are still identical twins who are different sexual orientations. and yes, it’s sort of true that allowing for any kind of biological difference means that people might assign abnormal status to it. but to be honest, that was going to happen anyway, and already does all the time. see: eugenics, scientific racism, the entire medical community’s concept of gender before like 1975, etc. trying to change our version of the story so that it won’t freak out the homophobes won’t work. we have to change the homophobes so that they quit freaking out.