The PG-13 rating has become so broad that it's kinda meaningless. And even when it comes to violence, I think that there's some weird distinctions. Like,
Daredevil &
Terminator 3 both came out in the same year.
DD got a PG-13 while
T3 got an R. But if you ask me, the violence in
DD is far darker and more disturbing and far worthier of an R rating. As far as I can tell,
T3 only got an R because of some swear words.
I think some of the confusion comes from the number. If I had my druthers, we would have a PG-15 instead of a PG-13, with a lot of the mass-appeal blockbusters like the Marvel movies falling into a broadened PG rating. Meanwhile, PG-15 would cover a lot of the stuff that's too risque to be PG-13 but so puerile that it mostly appeals to teenagers anyway; i.e.
Deadpool, South Park, most anything by Kevin Smith. (Not to say I'm above any of that stuff. I may be in my 30s but I'm fully in touch with my inner-15-year-old.)
I would certainly say that profanity gets far too much consideration in the MPAA ratings. The idea that there's some bright line between one "fuck" and two "fuck"s or between using it as an interjection vs. describing a literal sexual act, that's just ridiculous.
That being said, I think that most of Hollywood and society could stand to pull back on the profanity a little bit; not just the f-word but most of the others too. I'm as much a part of the problem as anyone but I think that it's something to be mindful of.
I agree violence, sex and language ratings should be separated, but there need to be some controls in place to stop kids from walking into a theatre and seeing movies their parents are against.
For the most part, I think that the MPAA does a good job of separating that. If your kids are mature enough to be seeing a movie on their own, they're mature enough to handle most anything that they would see in a PG-13 movie.
Also it matters if the nudity is integral to the story or if it was just put there for titillation.
Rule 34: If it exists, there's someone out there who finds it titillating.
I think that there are multiple dividing lines between different levels of nudity. Honestly, I think that most filmmakers are perfectly capable of implying nudity without actually showing any of the naughty bits. I don't think that breasts or genitalia have any place in a PG-13 movie. (I might make an exception for Graham Chapman's hilarious penis but that's it!

) If you're going to show the naughty bits, I would give it an R. I would only escalate that to an NC-17 if it's prolonged, explicit sex;
Blue Is the Warmest Color being a textbook example.