It's been a long time since I did any serious reading on human sexuality, and much of that was by late researcher John Money, who had some problems on his own...
Still, many of his ideas seem to remain in the paradigm, at least so far as I can tell by Googling...
One of the primary notions is that of the "lovemap". This is a psychological construct of an idealized mating partner. It's formed very early in life, based on parents, caregivers, siblings, etc.
It's no secret that men "marry their mother" and that women "marry their father" to a degree that's noticeable. Sometimes more than once...
We all seem to be attracted to a "type", and to find certain characteristics more attractive than others.
The same applies to "alterations" of the lovemap that go outside what most would consider normal and are classified as paraphilias. (Homosexuality is not considered a paraphilia)
People who have become inclined to "fetishes" (to use the common layman's term) are so inclined from an early age, and often will exhibit behaviors even though they do not recognize them as sexual. It's usually with the onset of puberty that these attractions become sexualized.
Money's idea was that one's sexuality is essentially in place by age three, and that it is pretty much fixed.
However, it's pretty obvious that it's more fluid than that. Obviously, on a level below that of clinical paraphilia, the things that we find attractive and interesting sexually may change with experience and time.
We know that many who are perhaps technically homosexual will drift back and forth over time and will "experiment" with the opposite sex. We had the acquaintance of an apparently-committed homosexual couple. One partner overtly effeminate, the other a "bear" as they say, an outwardly-masculine fellow.
That partner abruptly terminated the relationship and took up with a woman, even getting her pregnant. He told his estranged partner that he "wanted to feel like a man."
As noted above, there is a thriving "commercial" trade in highly-effeminate "T-Girl" prostitution... And most of the customers identify as straight. Hollywood stars have been caught up in such affairs, and will usually deny knowing that the T-girl was actually a man... Though it's glaringly obvious.
Money didn't address homosexuality to a large degree, but he thought pretty much as is apparently the current paradigm... That causation involves a genetic factor, certain hormonally-influenced changes in the fetus at critical developmental stages, and also environmental factors "as yet unknown".
Many homosexuals report knowing that they were "different" at an early age, often 6-7. Interestingly this is also the age when people report their initial attraction to the fetish items or scenarios they enjoy.