Several reasons why this has become an issue. Mostly it's the amount of space which was allocated to restrooms. And until recent times, the answer was "as little as possible."
Wasn't very long ago (to us old guys), that the men's room was basically a long trough where everyone sidled up like a bunch of little piggies. (The individual urinal with the privacy divider is a much more recent development). Yes, there were a handful of stalls, but (especially at sporting venues), most of the business was the re-deposit of beer loans -- so a trough worked just fine.
The architects were men, who (for some reason) had assumed that 10 men and 10 women needed the same amount of space. Or they just didn't care. Space is valuable. (And sometimes, the "extra" space was already being used for large "powdering rooms" for the ladies -- go see Radio City Music Hall.)
And, without being coarse, men can "prepare" for the event a lot faster (and earlier) than a woman can. And those seconds add up. And since all the women need to use the stalls, the occasional fellow who also needs the stall for a bowel movement is really at a disadvantage to everyone else. (Let's be honest -- anyone who is having a bowel movement at the theatre is having some kind of "unusual" timing (maybe related to dinner?) and it's very bad to be competing against a line of women who need to urinate.
The women at least have a choice of where to go -- they can wait at the woman's room, or they can crash the men's room. What choice does a man with an urgent need for the bowl have?
In Europe, most of the facilities are uni-sex, because there are no urinals -- you enter the bathroom, and there are communal sinks -- but every lav has its own enclosure. Nice idea -- but they take up lots of space, and stalls are harder to clean (and keep clean) than urinals, so there's that too.
For me, it's OK if a woman wants to use the men's room, as long as she accepts that she is going to see a lot of penises. (At the Paris Opera Bastille, the men's room closest to the hall usually has more women than men.) Of course the problem comes when there are so many women (who again take more time) that I cant get to a facility. And, of course, children are not welcome.
And since I am now a man of a certain age, I'm not in total control of what my endocrine system wants to do. Kudos to all the young men who can just point and shoot. (Wait -- your time will come!!) The appearance of unexpected events will always cause a significant delay in liftoff. It just is -- it had to be an old man who thought to post those ads over the urinals. Lots of time to read them.....
And sometimes I don't have the "flexibility" to wait. And peeing down my leg makes me very irritable.