Sex is weird too. You undress and make your self vulnerable and expend a lot of energy and risk catching a disease and then fall asleep. Either we do it for fun or to create a parasite that we have to take are of.
Most animals don't have clothes to take off and don't fuck at sleep time
Humans are odd, and I think our sleeping after sex is just that we've structured society to leave the best time for sex late
I never slept right after sex as a youth as the end of the educational day and when my parents would get home left only daylight hours with sufficiency privacy