Half disagree but I still love you. Their definitely is a gap in the literature for some of your points.
I mean
as a homosexual I can basically assert that you're
mostly right. There are some people who are just #BornThisWay, there are other people who are bisexual but something happens to them and for whatever reason they choose to only pursue one sex or lose attraction to one sex entirely for no reason. I think Nick is basically here but is likely more on the side of he would say he was bisexual but is actually gay which is not uncommon for gay people to do. This is especially true if they have trauma around a sexual experience with one sex or the other. I'm ready to die on the hill that something weird happened to him as a kid with women just because nobody hates one sex that much without something terrible happening.
There's something called the Kinsey scale that kind of explains this that is a used measure in Psychfagging .
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Basically, a zero is that your heterosexual and a six is that you're completely homosexual. But people who are bisexual or at least a little flexible tend to be somewhere in this range of 1 to 5. Some people who are at either the one or five range will identify as either homosexual or heterosexual depending upon their preferences.
Example: if you would not be in a long-term romantic relationship with one sex but
you would have sex with them you're probably here as a 1 or a 5 depending upon preferences.
I would honestly probably put Nick at a five but this is only because he's into dominatrixes that I'm going to
optimistically assume are not trannys (x to doubt).
I also want to say that there can be some fluctuations in preference over time just as how people's taste in partners kind of matures (

) as you get older. This is usually due to social factors. But could also just be people finally realizing that they're fucking gay and were blind to it because of heteronormative standards (aka, everyone else is straight so i will follow the crowd and not question it).
With homosexual women, it's not uncommon to have "late in life lesbians/bisexuals" because women don't really have an easy tell of attraction aka they lack raging Boners at the idea of hot girls.
On the other hand, those who are bisexual are usually gay and
coping (so says the literature.)
Bisexual women are more likely to be in relationships with men while bisexual men are more likely to be in relationships with men.
Tl;dr- nick gay haha