people are people but also some are "alphas" and some are "omegas". alphas are ultra horny and can impregnate anyone, omegas are ultra horny and submissive and can be knocked up by anyone. this is outside of the usual male/female thing so you got dudes getting pregnant.
big overlap with like werewolf fanfic or something, people go into heat and everything, also very big on incorporating characters from all sorts of shows.
it's odd taken as fiction but perfectly explicable when you realise it's just made to center a few kinks (mpreg, medically uncontrollable lust, domination/submission fex)