There is a documentary that examines the brains of actually gay goats. I guess you can not do that with people, so they have to use animals. There are (under microscope) visible physical differences.
So in this case homosexual orientation can be and probably is also in humans based on the physical wiring of the brain.
I imagine it is the same for other sexualities.
In my opinion being a zoophile is a developmental mishap. A dead end for that particular individual who is not going to reproduce. A mistake triggered by a variety of things.
But in all honesty I love being a mistake, because I get to suck on a fucking delicious dog penis.