All Women Are Prudes/Analysis
The trope did have some basis in Real Life (and in some places of our civilized Western world may still have). Not as much in women's supposedly innate (or conditioned by education to brainwashing levels) prudishness, but as a counter-reaction to the societal pressures around.
As long as a large percent of men felt due to early-life education and peer pressure their chances of getting social prestige are conditioned by having a female mate (or more), they would try to accomplish this by hitting on any suitable female around until they would get lucky, and afterwards would still do it, to preserve their self-enforced image of ladies' men. Even worse in societies where men are pressured into appearing macho to the public and worse still where men hold most positions of power in state or workplace hierarchies. But this would take for granted any female around, from supermodel to the ugliest troll, gets her fair share of approaches ...24/7, day by day, for decades. And not only the classic manner of approach, with flowers and love letters, but also the rude forms, including sexual harrassment at workplace and veiled threats if she does not submit. After some time, it becomes annoying enough. So the reasonable women are compelled to do something to get rid of this - either marrying the first suitor on the grounds that a married woman is harder to approach, or becoming prude as a nun, for the same reasons. This may also be one of the reasons for which lesbian women play the butch part - less for being attractive to other women, but to keep guys at bay.