Back in the before times, there was a movie review show, "Siskel and Ebert" that pondered this some. Why was Nightmare on Elm Street, hacking young people to bits R rated, while "Boys Don't Cry" NC 13 because you see Hillary Swank wipe her mouth after simulating oral sex with another girl?
A theory on why from a stringent Catholic game reviewer. I can game or watch violence and not feel violent myself. I do get a sense of vicarious catharsis seeing the bad guys getting their due but it is very different from feeling murderous myself.
Sex is different. You show a woman nude and I'm going to get aroused. For real. (Kind of why Freud says sexual response is what he wanted to study: that humans have genuine, physiological responses to sexual attraction, sometimes with no real stimulus. Read a good p#$n story sometimes and you're not even seeing anything: just thinking about it. )
To that staunch Catholic, that kind of attraction is, by itself, sinful. I'm not married to that lady, how dare I experience REAL feelings such as that? Others in power may have similar issues with this.
Why is male nudity showing penis so much more rare than showing a female nude? The woman's genitals are typically still hidden while the guy's penis IS his genitals. I do not know if Tinto Brass movies were R rated or not. They weren't typically actual p#$n ... but that women's genitals thing? Yeah. He went there.
Way too many people are offended by a penis
Even more people are offended by an erect penis
It also a way to dominate women.
It’s called the patriarchy and it’s been damaging men and women for thousands of years
I think we so seldom see one for reasons referenced above.
Also, are you writing that the penis is used to dominate women, so therefore we won't see them in media?
Another thing: many are writing of seeing plenty of nudity in today's movies. My own observation is, we're not seeing as much in the mainstream as we used to. I've read that with p#$n being so easily available, it has less and less impact on movie sales. You typically have to pay an actress more for her to appear nude in a movie, so, if that nudity won't sell tickets, why bother? (Halle Berry in "Swordfish" being the most extereme example of which I can think) 50 Shades not withstanding. That was a weird one that I'm betting disproportionately benefited from women wanting to see it. But yeah, I grew up in the 1970s and movies like "Carrie" were blowing us away. The new "Carrie". I think it was PG 13. EDIT: Nope. IMDB says it is R as well. I guess it is for violence rather than nudity as I'm pretty sure, in it, there is 0.