Gay males are defined by psychological, and even physical, stuntedness in childhood. Click on the category label "Gays" to see the wealth of posts supporting this view. It does better than the mainstream views that gays are feminized or hyper-masculinized.
They're Peter Pans who never grow out of the "girls are yucky" phase, which brings most or all of their other distinguishing traits along with it. Homosexuality in males is not just a sexual preference, it is a broader syndrome of dysfunction and depravity.
Now that I've established that to the best of my curiosity, I think it's time to move on to the two other non-hetero groups -- lesbians and bisexual women (there being no bisexual men: most are gay, some are straights trying to act "edgy"). Lesbians are too hard for me to spot in real life, and they don't stand out so much in society at large, so intuition will be lacking there. Let's start with the easier case of bisexuals.
Ideally the causes of bisexuality in women and gayness in men would be the same. Don't complicate the model if you don't need to. So, how does the psychological stuntedness model do with bi women?
The main thing that comes to mind is that they are the "wild child" type. They're not as infantilized as gays, though. They are more like horny and headstrong adolescents, fairly early adolescents, though -- middle school brats rather than college students. And they are bratty, just not as whiny as gays.
If the "gay germ" (in Greg Cochran's model) does something to halt or slow down maturation in the male brain, it could do something similar in the female brain. Same germ, similar effects, only not so extreme in intensity. Bi women tend to be intermediate between heterosexuals and gay men when it comes to deviance.
Why not so extreme? Women are less unstable than men. If there's any role of the X chromosome, women have a spare copy should one of them predispose toward harm. Male fitness is way more variable, so natural selection will only "see" the upper layers of the male distribution. Hence, males are the more disposable sex. Females have been designed to be more robust to such disruptions toward one extreme or the other. They are under-represented at both extremes.
So, if they got infected by the "gay germ," their maturation only slows down by half as much, and as adults they resemble middle schoolers rather than kindergarteners. That's if they got infected at the same age as males did. They could also just get infected at a later age, and the germ would have the same effect -- halt maturation to where it currently is. But then you'd have to explain why females got infected nearly 10 years after males did.
This post is off-the-cuff, so I'm not sure how broadly the model of "bi women as arrested middle schoolers" can account for the full suite of their characteristics. Having a high sex drive fits, and so does being willful, particularly toward authority figures. Below are more off-the-cuff thoughts on their traits that may or may not easily fit into this paradigm.
Masculinization. Bi girls are on the tomboy side in behavior, and as adults seem to have more masculine faces and perhaps skeletal structure generally (like narrower hips, broader shoulders). Testosterone levels peak in adolescence, are minimal for girls during childhood, and fall off very rapidly during their 20s and 30s as their body switches into mothering and grandmothering mode. Arresting their mind and body in adolescence would masculinize them as adults, although they wouldn't appear so relatively masculine during middle and high school, when the normal girls are also horny and headstrong.
Liking both guys and girls. This is their defining trait, so I might as well try to account for it. If their brain were arrested in childhood, they'd only like females. If it were arrested after menopause, they wouldn't really be attracted to anyone, though they'd probably prefer the company of women because they want a more peaceful and stable life, which men would disrupt. (Lesbians seem to be menopausal from an early age, but that's another topic.)
Female sexuality does seem more fluid and open than male, and early adolescent girls sometimes get practice kissing other girls at sleepovers or wherever. Girls are way more touchy-feely with each other in general at this age. This is intended, though, as practice for hooking a boy and marrying a man some day. It's only done with other girls because that makes it safer than doing so for real with a boy. In the case of bi girls, though, they don't out-grow this phase and mature into young adults who only prefer messing around with boys. Their maturation gets stuck in the phase of liking boys but feeling more comfortable playing with other girls, so that they're open for both.
That is a strong impression I get from them, BTW -- that guys to them are the real thing yet intimidating, and more of a sex object, while girls are more of a safe space where they can cuddle, make out, etc., and not worry about being judged by boys. It's very much like a sixth-grader would feel. It's not the other way around, where girls are mere sex objects while they romanticize guys.
More violent. This stems from having hormone levels frozen near adolescent levels. Anything that makes you think "hormonal" would follow from this general model.
There's more to look at, but those are a few random test cases that came to mind. I haven't given too much thought to this, so I'm not as convinced as I am of the "gay Peter Pan" model. Chime in with other well-known traits of theirs that need to be explained no matter which model it is.