Mine was also traumatic - my daughter needed a changing, and my wife sent me to take care of it. Challenge. Accepted. Only, in the fog of war I either did not notice the fold-out baby changing table, or it did not exist in this particular aircraft. Not finding the table, and with the door already closed, baby with dirty diaper in one hand, diaper bag in the other, I just laid her on the lid. I had to hold her squirmy self there with one hand to prevent her falling off, while the other busily took care of business. All the while knowing she, and me for that matter, was just inches away from “the hole”. My wife laughed at us when I got back to our seats with me in a full, sweaty lather.
Oh yeah, I do this too.
Same. I am of an age where the adage “Never pass up a restroom” has more meaning than ever. And that includes on flights.
IME those have only become commonplace in maybe the last 15 years. And probably first came out 25 years ago. So depending on how old that baby is now you may have had no hope of a table.
That exists - but it’s not why I try to avoid needing to use the restroom on a plane. It’s a combination of not always having an aisle seat and something that might only happen to me - I have trouble urinating in a moving restroom. Bus, train or plane.
I’ve been successful on a couple of short flights - but 2 hours is about my limit.
Oh, for sure that between the cramped space, the less than clean conditions, and the motion, and the potential for unexpected major motion, there are a lot of ways aircraft lavs are an offputting experience.
The question becomes, as noted by some folks upthread, just how much other forms of inconvenience and / or discomfort they’re willing to take on in exchange for successfully avoiding the lav.
Lotta individual variation in those tradeoff decisions.
Many times. Even did the baby diaper change in one. My method of diaper changing always involved spreading a changing blanket on my lap and changing my son’s diaper there anyways, so it was really no different for me!
And the doors aren’t exactly hidden…everyone, or many, at least, would see you both going in and know what was going on.
I don’t know how often you fly recently, but these days nearly 100% of people have their face buried in their phone / tablet. Ain’t nobody looking up and down the aisle.
Besides, on most narrowbody airplanes the lavs are at the back. The FA’s back there by the galley might know, but nobody else will.
If you’re trying to be discreet, just have one person go in while the other seems to be waiting their turn. But don’t lock the door. Then maybe 30 seconds later, when anyone who had been randomly looking up is once again engrossed in their device, the second person steps in and then you can lock the door and get your quickie in private. No one person saw both of you go in with no one coming out in between.
Do the same two-part maneuver on the way out and you’re golden. Unless somebody else has arrived to wait in line while you’re both in there. Busted!
Thought I mentioned how I try to avoid it - I use the airport restroom at the last possible moment , like after boarding has already started and I drink as little as possible before the flight.
I’m a tall, but relatively thin guy, and most of them give me more room to stretch than my seat does. I can’t imagine playing around in there with someone though, not even back when we were more lithe.
And I usually go at least once, and I try to get an aisle seat so I don’t have to climb over anyone to do it.
I’m tall and pretty big. 220lbs. Not really fat, but not thin. I get the aisle seat too. My wife is petite, she can get around me. I can’t stand up in a seat by the window. No way.
And being tall, I can help people get their bags from the overhead compartments.
On any long overseas flights I’m going first class. My vacation starts when I board the plane. It’s insanely expensive, but no, I just can’t do coach. I really can’t. I would seath for 10 hours and land. Not a good way to start a vacation.
Oh, my, you’ve given this a lot of thought! 
Yes you’re probably right, no one would pay attention. I will say that I’ve often sat right at the back (I’m cheap!) and the lavs are right over my shoulder… I think I’d notice. But I’m nosy…
Remember, I used to be in that line of work. See here from 20 years ago:
That whole thread is entertaining, not just my one post in it.
On Southwest we get early boarding so we can both get aisle seats across from each other. This was especially important before my wife got her new knees.
As for first class, we went from SFO to Amsterdam coach, and never again. So, I’m with you.
I don’t sit there in discomfort. I’ve just never had to go bad enough to get up, walk down the aisle and cram myself into that shoebox of a toilet. I simply go before I board or after I arrive. No discomfort. Well, that incident on descent into Minneapolis notwithstanding.
Sorry, this isn’t something you can train for – either you got or you don’t! 
However, I can offer a word of advice. Alcohol is a diuretic, so needing to pee more often when drinking is not only the result of the extra liquid, but also the diuretic effect. So lay off the booze if you don’t want to use the lav!
Is anyone else… afraid… of airplane lavatories?
At a baby shower once, one of the silly questions for the guests was “Name one thing you’re deathly terrified of”, and without hesitation, I wrote down “Airplane bathrooms!”. And I wasn’t the only one who wrote that!
I’ve used many, many of them, but still, every time, their crazy loud WHOOOOSH gets me. I can’t help but wonder what would happen if I fell in… would I get sucked right out of the plane? Ground into a pulp and then sucked out of the plane? The giant ball of solidified poop from Joe Dirt is forever seared into my nightmares…
Oh, sure. Many times. The best event was one of my wife’s visits to the bathroom. She had our then cat in a carrier in the cabin with her and decided that it would be nice if she went in the bathroom and let the cat stretch her legs. The cat saw opportunity and would have disappeared under the bathroom counter had my wife not managed to grab the cat’s tail as it was disappearing and drag her back out. Can you imagine what the conversation with the crew would have been like?
I always go first too. 6’3" tall 220 lbs. I can’t even cram my size 13 feet under the seat in front of me. That is discomfort. And a bad way to start a vacation. I much prefer “Another Gin and Tonic please”.