Carrie having her period in the shower in the movie Carrie. Not sure how respectable it was though.
And the Lifetime Achievement in the Field of Urinary Health Awareness In Film Award goes to…
In Semi-Tough, as Jill Clayburg and Burt Reynolds are in the seminar run by Bert Convey trying to get “It”, an enlightened woman tells the group that “I’ve just peed all over myself.”
What I want to know (not that I actually want to *see *it–I just want to know) is whether any mainstream movie has ever actually showed somebody taking a poop. Not the sound effects, not the aftermath–the actual activity. We’ve seen people vomit, we’ve seen them pee, but has anybody been brave or disgusting enough to show an actual, in-progress (non-animated, so leave out South Park) poop?
Dune - urine recycled to drinkable water.
Leaving Las Vegas - she pees with the BR door wide open.
Doc Hollywood - they pee to scare the deer away.
As it’s a crude movie, it probably doesn’t count for the OP’s list; but there’s a pretty vivid scene in Harold and Kumar Go to White Castle where two dainty British girls are playing “Battle Shits”.
Demi Moore and Woody Harrelson carry on a conversation while she’s sitting on the toilet. I’m guessing this was to show us what a close clouple they were or something?
In Along Came Polly there’s an excruciating scene where Ben Stiller has the runs after eating Indian food. And even though urinal scenes aren’t rare, the same movie shows Alec Baldwin putting his unwashed hand on Stiller’s face after using the urinal. I thought that was kind of a nice spin.
And I can’t think of a specific example at the moment, but surely there was something in Borat that fits the OP?
the movie S.O.B. deserves some sort of award for this – one character performs , against his will, just about every bodily function at least once – he burps, farts, pees, poops, and throws up. He might do more, but I can’t recall.
Both Brad Pitt in Fight Club & Michael Douglas in War of the Roses pee on food that’s about to be served for dinner (the former in a restaurant, the latter at a private function).
Treat Williams public urination in Hair
#30 posts and no one’s mentioned “Trainspotting” yet ?
You know the scene. That’s right: eeeeewwwwww
Flight of the Intruder had an ongoing subplot concerning the USS Enterprise having a vigilante aboard calling himself “The Phantom Shitter”
The heros finaly catch him late one night as he urinates in the XO’s coffee cup. (We see the cup between his feet and the urine pouring into it, from the POV of Cool Hand and Tiger Cole, hiding under a table.
We Were Soldiers also had a scene where a group of soldiers, led by Colonel Moore, piss on a battery of overheated mortars to cool them off enough to fire again. Same type of POV shot as in Flight of the Intruder, only now presumably from the POV of someone lying down trying not to be shot.
The Full Monty has a scene set in the mens toilet of a club during Ladies Night - a bunch of women are in there using the facilities. One of the women uses the urinal, to demonstrate that she is as capable as any bloke.
Si
I cannot believe someone beat me to that one.
There’s a scene in “Major League” where someone takes a contract, tosses it on the ground, unzips his fly and let’s it out to express his sentiments about it. They don’t actually show him urinating, but his hand movements and the sound effects suggest it.
In “Revenge of the Nerds II”, Ogre urinates for a total of 1 minute, 45 seconds ( cite).
Charles Martin Smith’s character in Dances With Wolves drinking large amounts of tea and marking his territory (while singing “I am the modern major general…”) to see if the wolves would respect his boundaries.
“Request permission to relieve bladder!”
“The man has got to go.”
I think you mean Never Cry Wolf, which I specifically came in here to mention. One of my faves.
Another: Fandango - pissing in the overheated car radiator.
And of course, Babe. 
Some foreign examples:
Wages of Fear - 3 of the drivers pee together off a cliff but exclude the 4th driver from joining their “male bonding” moment.
Kings of the Road - In a single uninterrupted take (a long shot), actor Rüdiger Vogler walks down to the base of a remote sand dune, drops trou, and takes a very real dump right onto the ground (no fakes or cuts). Great movie, btw.
Holy crap, you’re right. I can’t believe I screwed that up. One of those movies I love and the other I loathe - you can probably figure out which is which.
They also did this in Red Dawn after Patrick Swayze’s truck had been shot up.
I’m not sure if it is true but the commentary track for Alfred Hitchock’s Psycho mentions that this is the first time a toilet is seen in a movie. (I’m guessing a hollywood ‘code’ era). Janet Leigh doesn’t use it, but the shower.