I first noticed this around 1984. Since then
this symbolism seems to have become universally understood and accepted. Who originated this and why?
Don’t know the who, but I’d say the why is pretty clear, women have curves and men have angles 
Why were a circle and a triangle chosen? I would guess that they were chosen because they are two geometric symbols that are easily recognizable and obviously different from each other. They are in use everywhere because they are International symbols. I don’t have much more information to give you, except this.
[list=a][li]Those symbols are required by law in California. “California: Legal rights of persons with disabilities”[/li]
Buildings open to the public must have signs posted which indicate the location of restroom facilities accessible to disabled persons. Signs on restroom doors must be in the shape of a circle for women’s restrooms, a triangle for men’s restrooms, and a triangle interposed upon a circle for unisex restrooms. (Cal. Code Regs., tit. 24, pt. 2, § 3105(A) (b) et seq…)
[li]An exact description of the symbols (from another site):[/li]Signs & Identification: The International Symbol of Accessibility shall be the standard used to identify facilities that are accessible to and usable by physically handicapped persons. On doorways leading to men’s sanitary facilities, an equilateral triangle 1/4 inch thick with edges 12 inches long and a vertex pointing upward and on women’s sanitary facilities a circle 1/4 inch thick and 12 inches in diameter. These geometric symbols shall be centered on the door at a height or 60 inches and their color contrast shall be distinctly different than the color of the door.[/list=a]
That doesn’t really answer your question, but I thought the information might interest you.
That’s interesting. I didn’t know it was
codified in the law.
Well… There use to be a cresent moon on one outhouse and a star on the other. The Moon representing women (Greek myth) and the Sun representing men (Greek myth again). Maybe the circle and triangle are just new-age versions of that.
“Clatu, Verrata…nector?..neck-tie?”
I did. It was the Summer of '73 and Johhny Mac and I were driving cross-country. We stopped in a gas station in Sheboygan and Johhny couldn’t tell where to take a leak. One door had a Ford sillhouette on it; the other had a Chevy. I told the attendant, “You’d better get some standardized signs, mister, with a circle for women and a triangle for men.” Then we shoplifted a fifth of Old Grandad and hit the road.
The next year, they were everywhere. Should’a held onto the patent.
I’m your only friend
I’m not your only friend
But I’m a little glowing friend
But really I’m not actually your friend
But I am
Da Ace: Whoa dude! That was you, man? Heavy.