"Why men fall asleep after sex"
Because we're done!
I'm not saying this tactfully but the world's oldest profession wouldn't be a profession if they fell asleep after the first customer.
This was explained to me more properly in the book "Anatomy of Love: A Natural History of Mating, Marriage, and Why We Stray", by Dr. Helen Fisher. She received her Ph.D. in Physical Anthropology with a dissertation on the evolution of human female sexuality.