I would recommend reading https://en.m.wikipedia.org/wiki/Sex_at_Dawn book. While getting lots of academic criticism, it describes logically (and criticism for that was the most minor) well why penis has a form it has sex last that amount of time when differs for men and women.

To cut it short our ancestors species females had sex with several males (and vice versa), where evolutionary not the strongest male would reproduce, but whoever sperm will get to the ovum first. So penis is a pump which should help to remove as much sperm of previous males as possible, (that’s why some time for sex is required), ejakulate as strong as it can, than soften and loose shape to stop acting like a pump. Also there were spermicids found in substance males ejakulate in the end, which might be to minimize next male chances.

Partly book is very questionable, but it has some valid points and quite fun to read. No idea what is true or not =)