I have a theory. Readers can see what they think of it. Let it be known that this is just conjecture.
My theory is developed partly in response to the fact that blowhard feminist types, including male egalitarians, will sometimes bring up the fact that some girls are tomboys and nonetheless turn out just fine and use this to defend raising boys "gender-neutral," encouraging them to play with dolls and imitate Mommy, and the like.
It seems to me that the problem with this reasoning is that there is a major asymmetry between the situation of little girls who do or want to do stereotypically masculine things and little boys who are encouraged to be effeminate. The bottom line is that it seems that tomboyishness in a girl is less likely under natural circumstances (an important caveat) to translate into gender confusion in an adult woman than effeminacy (by which I don't mean simply not being athletic) in a boy.
Now, I hasten to emphasize that "under natural circumstances." If a tomboyish girl is surrounded by perverts and their enablers who teach her that many people just "are" lesbians and who encourage her to think that this is what her tomboyishness means, then that may be what happens. But absent this, she may just run around like a little hoyden in her youth, maybe get into swimming or become a triathlete when she's older, and nonetheless get married and be quite feminine. Ideologically she might or might not be a feminist. That's not so much what I'm getting at. I'm rather trying to say that tomboyishness in a girl doesn't have much of a natural tendency to turn into actual lesbianism, transgenderism, or general psychological gender confusion.
If, on the other hand, a little boy doesn't bond with an older man who is a mentor or father-figure, if he's raised too much in the company of women, if his mother stifles him, and especially if he's encouraged to think of himself in distinctively feminine ways--e.g., to imitate mothering behavior in his play or to wear female clothing--this can spell big trouble for his gender identity as he gets older.
These are all, of course, outrageously anecdotal generalizations, but they seem to me to have truth in them.
Why this apparent asymmetry?
Here's where my theory really gets wild: My theory is that this asymmetry arises in part from the fact that what we think of as distinctively masculine activities are in many cases the epitome of human activities. For example, training one's body and being in good shape, keeping animals or training animals, having dominion over nature, being sharp and analytical with one's mind, or even engaging in intelligent and trained fighting against evildoers. These are all things that are done or, in the case of fighting, can be done in an especially human way that represents mankind. Therefore, it is to some extent understandable that girls want to engage in them, to make up stories in which they are a boyish hero riding a horse and smiting bad guys, for example, or to construct a beautiful argument or win a glorious chess game.
The truly distinctively feminine activities are, by contrast, more narrow in scope and in a sense more characteristic of what mankind shares with the animals. Here I'm thinking especially of bearing and nurturing children.
There is, of course, nothing wrong with a man's being a good father, helping his wife with the baby, and spending time with his children. In fact, that is all extremely important. Mankind has been designed by God to have one of the most long-term father relationships of any creature in nature. But fathering is not mothering, and the instinct to mother-love is to a very large extent shared across the spectrum of mammals and even birds. Of course this isn't in any way to deny that human mothers have anything distinctively human about them. It's just that, on my theory, for a man to try to behave like a woman and a mother and especially for a boy to try to behave like a girl is for him to mess himself up in some fundamental way, whereas the same does not seem to be always true for a woman who tries to "argue like a man" or a girl who tries to "play with the boys." It seems that the female is more resilient to that kind of role-playing than the male, and this might have something to do with the fact that in many ways a girl role-playing at being masculine can be doing something uplifting and something that reflects admiration of distinctively human characteristics whereas a boy role-playing at being feminine is doing something that degrades his identity.
This also seems related to the fact that a girl can wear pants without necessarily being masculinized while a boy cannot wear a dress (and no, I don't mean a Scottish kilt) without being feminized. No doubt I have traditionalist friends who will disagree with me about the first conjunct of the previous sentence, but by observation I think it is obviously true.
I don't have this all very well-worked-out, as you can see. There will be lots of counterexamples to anything of this kind that is overgeneralized. For example, a woman trying to act like a man (or like her concept of a man) in a management position is going to end up inevitably being an odious bully, which is degrading to all concerned.
I will be interested to see what thoughtful ("thoughtful" here means among other things "not known or obvious members of the manosphere") readers think about these odd thoughts.