I’d always thought that it was a ‘shortcut’ signal that the woman was supposedly highly aroused. Coudn’t say that outright at the time (1920s), so used a visual cue. Or maybe not…?
I figured that was the idea. It’s a sign of submission as opposed to putting a leg back to keep from falling over. Also it highlights the actress’s legs in a movie. I don’t know how it used to be but a couple standing straight up and doing nothing but engaging in a passionate kiss with one leg raised doesn’t seem to be the typical way people make out.
It’s a visual cue only to those outside the couple. The man is getting plenty of cues already and probably wouldn’t even notice a leg raise. The movies invented it because the perspective has been shifted to an outside viewer who can’t feel the experience. You might even say that it’s a euphemism for an erection.
Real life woman don’t seem to do it because it fills no need and is not a natural or comfortable movement. Movies and photographs are filled with artificial poses to make them look better. Women in real life seldom stand in the positions photographers pose their models, nude or otherwise. Nor do woman always pull the sheet up over their breasts when they sit up in bed after sex. I don’t have any problem believing this is yet another movie contrivance.
LOL!