Real answer
It's pretty much biological or environmental, we can't study it more so we don't know how much of each or how many in each category if they are separate.
However, Master's and Johnson ran a study on foot fetishes that has some possible insights.
They hypothesized that all or most sexual preferences were learned through association, including fetishes. Feet, not directly being involved in sex, must be one such learned fetish.
So they took guys that only liked normal solo girl porn, and validated this with devices that could measure arousal of an erection and they were to come into the study periodically to masturbate to a defined picture set.
For a group of them they slowly made the porn more about legs and whole body and then showing feet, then including ones that had closeups of feet and so on.
By the end of the study, everyone in the treatment category had developed a foot fetish. Most thought they just liked feet and denied the study having such an impact despite foot fetishes being rare at the time.
One guy even got to the point where he masturbated exclusively to photographs only containing a shoe. Despite being told why this happened and how to undo it, he said he just liked shoes.
Science can create a shoesexual.
It stands to reason that if you are really into gay porn, then start going backwards to bi, orgy, or pegging porn until you can jack it to girls. It's possibly all learned associations.
I think this theory also explains the massive rise in trap porn. Couples porn and blow jobs were very common porn types, people started associating dicks with orgasm and feminine features. Traps are the embodiment of that.
Healthy heterosexuals should stick to solo girl or lesbian porn as to not create any degenerate attractions.