Probably the evolutionary arms race between rapey males and females. Males have long corkscrew penises that inflate rapidly during copulation to fill the tract of the female. Females, what with their desire to choose mates and all, have evolved corkscrew vaginal tracts with dead ends and stuff that spirals in the opposite direction. Forced mating frequently ends without insemination, as a result.

I am genuinely unsure of the mechanism by which successful mating actually happens, considering the anatomical arms race and all (maybe it’s like dolphins, which have a similar problem. Female dolphins have a false vagina and can choose where the male’s sperm goes by changing her tail position), but they seem to be able to reproduce, so it is what it is, and I’m not super keen to look it up at the moment.