I think human excrement and boogers may be less common.
When Terry Gross interviewed the director she asked him about the scene. Director said that as they were filming the scene, “the event occurred”; he (Director) stopped, asked the actor if he was okay with it, actor said yes, so they resumed filming.
The first thing that came to my mind was John Ritter’s Skin Deep, wherein two erections are depicted onscreen, but in pitch-black darkness and using glow-in-the-dark condoms.
Jack Black’s erection saved the day in Tenacious D and the Pick of Destiny. He credits cock push-ups.
I was thinking about that, but it’s been a long time since I saw that movie.
There’s one in Irreversible … but you probably don’t want to see it. The film is notorious. It tells a story of revenge, Memento style, where one of the first scenes is two guys beating a guy’s face in - literally - with a fire extinguisher. But the scene that shocks the most is the ten minute long, un-cut (as in, no edits - one camera for ten minutes) brutal rape scene after which you see the rapist’s erection flopping around for a second when he’s done.
Don’t search for it if you don’t have a strong stomach.
Back to the specific question in the OP, I’d say that no, actual buttholes are less seen than erections even. Sure, you see butt cheeks pretty durned often, but the actual round brown? Honestly can’t think of a single one.
I didn’t know this until years after I had seen it, but in Animal House, the scene where Bluto falls over backward on the ladder he uses to peep into the sorority house was supposed to be caused by Bluto getting a boner watching the women.
I guess I was just too innocent to put it together.
I see a lot of movies; 50-100 per year. I’ve never seen a vulva, or any part near it, other than pubic hair.
The Dreamers.
Inserts. Veronica Cartwright gives a brief spreadshot.
Mad Dog Morgan. (1976)
Pink Flamingos, if that counts.
I’m going to be seriously impressed if anyone comes up with too many more rusty sheriff’s badge sightings–it’s encouraging that both examples so far are from fairly obscure movies. 
If animation counts, the Netflix animated series F is For Family featured scene with a kid with an erection under his swim trunks at a public pool. I guess you could say it was partly for comic effect, but the main point of the scene is how embarrassed the kid is about the situation, so really the effect of the scene is that you end up feeling sorry for him rather than laughing at him.
I feel actual buttholes like actual erections are probably NC-17 worthy material.
However there’s a ton of movies from the 70’s that had nudity and didn’t seem to really care if for a half-second somebody’s butthole was exposed during a nude scene.
Oz, the HBO series about Oswald Maximum Security penitentiary, had all kinds of penises waving about, including Luke Perry’s in one scene, but I don’t really remember any that were standing at attention.
I don’t know how Americans feel, and I know it’s a sterotypical “generic Southern” accent, but I felt Walliams was way better than the usual British comedian at an American accent, there.
Things seen less often than erections in modern movies?
I’m going to suggest: Aliens from Neptune. Haven’t seen one of those in a long time. The last (and only) one I recall was a brief cameo in the movie version of My Favorite Martian.
The first movie that came to mind, though I had to look up the title, is "La Dernière Femme" from 1976, with Gérard Depardieu and Ornella Muti. I saw it one time as a teenager in the mid-80s on regular German TV. I may have watched it in the hope of seeing some of Muti’s flesh, but what I got instead was Depardieu running around naked for virtually the whole film, and at least one time sporting a clearly visible erection, no body double. I don’t remember much else, but it was a weird movie. That was the first openly visible erection I ever saw on TV, and I can’t remember having seen another one since then.
all in the Family says they were the first show with audio of a toilet flush