Someone needs to go bleed Brutus.
Like you, my wife sometimes objects to my (within -the-household) announcements, but I counter that she really does want to know:
- For about how long I’ll be occupied;
- Whether she’ll need to stay away from the bathroom for a while, lest she suffer olfactory distress
If I’m really sick, I might end up “praying to the porcelain god.”
My Granddaddy used that one, and Ms. P has gotten to hear me use it in the last few days. Beavis and Butt-Head would occasionally use it as well.
‘Ride the porcelain bus’ for diarrhea.
If we are hanging out on the porch/patio/living-room and I leave to go to the loo, I’ll say, “hold my calls”.
I wake up in the morning, and go into the bathroom for my “morning constitutional”.
If I use a euphemism, I’ll usually say “I’m going to the small room for a minute.” I mean, no one needs further detail. I’ll admit that they may appreciate a heads-up if there’s a possibility the atmosphere in the room will be left noxious, but I figure they’ll assume that anyway.
Of course, “small room” doesn’t work if the location is a large public facility with restrooms designed for numerous users.
Just the simple Poop & Pee
Barfing: Selling Buicks or talking to Ralph
I use that one too. Sounds classy!
I say ‘urinate’. If I really have to relieve my bladder, I say ‘I have to urinate like a thoroughbred!’
And I say ‘defecate’.
People don’t seem to appreciate my use of those words.
Actually, now that I think on it, I usually say that I’m going to “step into the shitatorium for my morning constitutional.” Less classy. 
If I’m in polite company, I’ll excuse myself to “wash my hands.” However, if its me & the missus, I have to “go save the world.” which has roots in a Jake Gyllenhall movie.
Tripler
Saving the world, multiple times a day, since 1977.
I simply say “I have to use the litterbox” ; no specifics included.
As a kid we all thought it was hilarious to say “I have to drain the main vein” (even us girls).
I believe this is what James Madison used to say.
mmm
I once said this to my homeroom teacher, because my mother had written me a note to excuse me for an appointment. He wanted to know what the appointment was. Well, I was going to see a man about a horse. No lie. But the teacher wouldn’t believe me. My friend, who was coming with me, just kept laughing at our teacher (a good guy) and saying “Really, that’s what she’s doing.” So, he did let me go and I went horseback riding. 
Depending on the folks around me, either I have gas or someone farted in my pants.
When I have to pee, I’ll say, “I have to go drain the snake.”
When the Li’l wrekker was about 3yo my Daddy was all about teaching her Japanese curse words and taking her to the track (I was with them, creeping up in the back)
He was routinely sending her to get beers outta the fridge.
I finally sat him down and told him to lighten up on her. She would have to go to school and be in public soon.
So getting a beer turned into “Go get Diddy a blue milk(Busch beer)”
The track turned into “Petting the ponies”
Japanese and other curse words I had to threaten her to never say those words.
Her Kindergarten teacher ask me did we have horses? Who taught her Japanese and what the heck was blue milk? She asked for it everyday in the lunch line. (Yep, he was giving her sips of beer. She was soused as a toddler)
I said, “Jeez Miss Muckymuck, she must have picked that up during recess.”
Yep he was a character.
I remember in childhood that women excused themselves to powder their nose.
They probably did touch up their makeup after using the restroom.
I remember seeing this in old movies too.