The line between male and female is thick and unmistakeable, drawn clearly not by individual consciousnesses but by society-wide repetition. Lines cognized and acted out nonconsciously. They function without being made explicit. We learn language first by using it. If you're lucky/unlucky, you can learn different things by analyzing it later. But children understand grammar, specifically, how to use it, while lacking the ability to analyze and explain it. The gender distinction is learned by using it.
Undiluted (thankfully it's often diluted) the distinction is this: boys need to kill everything and replace it with themselves...or die. Dominance or emasculation, penis or no penis, boy or girl, life or death.
The boy, having become this message, needs to reconcile it with his asymmetrical relationships with women. It's humiliating for a hyper-gendered boy to be dominated by a woman. So he comes up with narrative fixes, outs, explanations where he's not actually being dominated. He believes these, tenuously. They're pretty obviously bullshit and reality is always knocking.
He hits puberty, "knows" he needs to fuck a woman in order to be a man. This is a huge change, and a huge challenge. To request this fucking is to risk rejection. Rejection is emasculation and death. A simple binary, the line around it in the worst cases impermeable. Santa Barbara killer Elliot Rodger went to great lengths to put himself in a position to be accepted without risking final rejection. He never, apparently, made any kind of advance on a woman. Instead, he spent great amounts of time and money on his own appearance, went to places he expected to find good-looking (blonde) women and hoped they noticed him. He never "put himself out there." He left himself an out. Maybe next time. No one actually said no. Or, as Jim Carrey's Lloyd Christmas said in Dumb and Dumber to his love interest's assessment of his odds of getting with her as one in a million..."so you're saying there's a chance."
This is really amazing. A single woman saying "I'm not interested" or "go away, creep" or laughing was far and away the most powerful force in his life. No, the possibility of that.
The MGTOW (Men Going Their Own Way) phenomenon is an obvious out. "Hey, women-we-want-to-fuck: we don't want you anyway. You are not rejecting us." It's understandable, I mean, given the above. They're trying to go pre-puberty. Back to a time when they could less implausibly claim to be not-dominated (where non-hyper-dominance means being dominated -- again, it's a pretty strict binary). Before the need-to-fuck-to-be-a-man challenge. They'd been talking about how they could easily dive off the 10 meter platform but now they're up there. Do it! *Tries to find a way out, can't, shoots a bunch of people instead.* That's Elliot Rodger.
Rapists take away the chance for rejection. This is the point. Rape is the most extreme form of masculinity. (I worry a broken rapist somewhere is thinking "yay I make sense! yay, masculinity!" but it should be clear this is a condemnation of masculinity.) OK, outside the parentheses, still worried this could be taken the wrong way, given certain positive associations with masculinity, but really just want to end this post. OK.