Would you say that the conversion of TV from broadcast/cable to streaming has resulted in a lot more nudity? If so, why hasn't Internet porn reduced it?

Here's the point I've been circling around: the availability of Internet porn does not adequately explain why depictions of sex and nudity in movies have gone down. It's the first idea that pops into peoples head, but it doesn't quite fit. What does is the rating system. Somewhat with the introduction of PG-13, and more dramatically so with NC-17. "This Movie Is Not Yet Rated" goes into this in more detail, but I'll lay out what it's getting at.

If you go back to the 1970s and '80s, you have PG movies with nudity. "Airplane", released 1980, had a quick flash of boobs along with an extended blowjob joke. "Superman", released 1978, had Superman as a kid climbing naked out of that pod. Expressly non-sexual, but nudity none the less. Today, Airplane would go straight to an R rating for that flash of boobs unless it's from a director like James Cameron, who gets to pull strings and do whatever they want. I don't think you could do the Superman bit at all.

You also have some R rated movies at the time showing extended closeups of the faces of women in sexual pleasure. This has almost entirely disappeared from all mainstream movies. Liv Taylor's character in "Jersey Girl" (PG-13) talks about masturbating, and that was scandalous.

Then PG-13 shows up in 1984 in response to movies like "Indiana Jones and the Temple of Doom" pushing PG too far. When that happens, PG becomes the older kids movie rating, and PG-13 is supposedly for teenagers. Except, now you can't really do scenes like Temple of Doom did and still be PG-13, either. Too much blood. Plus, you can't have nudity except maybe the odd butt (usually male), again with the exception of being James Cameron.

Also, you get one F-bomb in PG-13 movies. It has to be stated in anger ("fuck you") and not in reference to sex ("Should we go home and fuck each others brains out"). This isn't an official rule anywhere, but even people outside the industry have picked up on it.

So now you can have James Bond shooting up tons of baddies as long as you don't show any blood. The same movie will also go to great lengths to carefully conceal the lead actresses' nipples at all times.

This gets much worse when NC-17 comes along. This was an attempt to rebrand the X rating, which tended to be associated with outright porn. "XXX" was never an MPAA rating; the porn industry adopted that for itself, but the association got stuck. So hey, surrender that idea to porn, change X to NC-17, and now we can make "serious" movies with lots of sex.

Showgirls then completely bombs.

What happens next is that NC-17 is used as a bludgeon by the ratings board. Do what we say, or else we'll rate you NC-17 and most of the theaters won't even show your movie. There's a bit of psychology going on here where the ratings board wants to feel like they have a say in the movie itself. This has sometimes resulted in directors deliberately putting in stuff they know will never pass, then it gets flagged by the ratings board, they drop it, and the ratings board gives it the OK.

You can't always do that, though. Directors won't bother shooting a scene at all when they think the ratings board will nix it. Nudity has become nearly absent from R rated movies altogether because of this, and it's a very brief flash if it's there at all. One exception being Wolf of Wallstreet. Directed by Martin Scorsese--another director who has enough pull to get whatever they want. Anybody less than an S-tier director doesn't get to do that. That movie is now 11 years old, and I'd challenge you to find another R rated movie with that much nudity and sex that's been produced since.

Violence in R rated movies hasn't gone the same way, because the ratings board members don't care as much. They're largely Americans (as far as we know; they were when "This Movie Is Not Yet Rated" was produced), and American culture is stuck in a mindset that violence is less bad than nudity. Also, Showgirls was known for sex, not violence, and that's the sack of bricks hanging over every R rated movie director.

So in a perverse way, the opening of PG-13 and NC-17 ratings have actually reduced artistic expression, not opened it up.

Streaming evolved in a totally different way, and isn't subject to the same incentives.