Through an incredible mixture of lack of education (or understanding on my part) and very likely "toxic male" disgust at regular human functions when performed by their feminine counterparts, I was unaware of menstrual cycles until I had a girlfriend for the first time. Very early 20s. I never revealed this information to her either, but I was internally fascinated at this entire "secret" held by ~50% of the population that I had no idea about.
I know people will probably think I'm exaggerating, so all I can say is "I'm not." I didn't know how tampons worked. I had no idea really how often women have their periods. All I knew was vaguely like "Women bleed sometimes and it causes them to be..." well, fill in the blank there. Stuff I had picked up from popular media, basically.
I have to credit her (my ex) with dealing with what was surely a nearly comical amount of ignorance. Probably something women who date men are used to. She had to teach my dumb ass all about getting pregnant (the times women can/can't mostly). And things like "It's messy. There will be blood. Possibly everywhere."
Thinking back now, it really is fucking hilarious that I had no idea. I promise you with 100% certainty that there are a ton of teenage to young 20 year old men right now, possibly even reading this, who also don't know or have the vaguest possible conception of menstrual cycles. I don't know what more kind of education would be required. Maybe this is something guys, especially ones without sisters or young cousins around, just don't encounter until they have a steady girlfriend and reality becomes unavoidable. I kinda get the impression though that there are tons of 40, 50+ year old men who have whole-ass wives and maybe daughters who still refuse to learn about periods and what's going on. They just put up a mental block and go "nope! Don't talk to me about it! Just let me know when it's over!"
To really tie this all to the main question though, that's the type of "harm" I can say I had. Just this general avoidance of bodily functions being discussed by men allows us to be, well, full grown men and still totally ignorant of things we absolutely need to have a decent understanding of. When I hang out with or am sort of just existing near my girlfriend and her friends, or she tells me what they discuss, there's a lot of openness and frankness when it comes to "gross" functions. Men will make crude jokes, but the dynamic is different it feels like. I don't know how to really word it, but basically women are forced to "grow up" and don't get a choice about it. Teenage girls are thrown to the wolves. While men get to be boys forever. It's just acceptable to not take things seriously and not know elementary things. In fact knowing things and showing some level of empathy is seen as "being woke" (or "gay" in the bad sense of the word) or whatever. You're pretty actively discouraged from caring about women's issues.
This shit affects men, too. Like abortion being illegal in places in the US 100% harms men. Not to make this all about men, but for the purposes of this discussion. Men have to deal with the consequences of a pregnancy as well. Especially if he and the mother don't want a child... yet she can't do anything about it. Or if you do want a child, both parents let's just say, but the pregnancy has complications, needs to be terminated, but no doctors will touch her due to ambiguous laws. And she dies. I'd say that affects men a great deal.
This culture of acceptable willful ignorance by men prevents many men from ever being "men" in a way that I'd define it anyway. Basically someone who isn't afraid to stand up for and defend the marginalized and "weak" in society. It's created entire generations of emotionally stunted, ignorant, and frankly, just being real, pathetic boys in man-sized bodies. You can listen to any right winger now days, and it's literally the mind of a 14 year old boy inside of a 35 year old man-body. Ex: Andrew Tate or Ben Shapiro (I'd posit neither of these man-boys know how "pussies work" still to this day, which I think probably proves enough of my long winded comment. They don't need to. If they ever expressed the knowledge that they do, they'd get mocked by their right wing peers for caring)