system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does sex take so long? Why doesn't the male simply insert his penis, deliver the goods, and then withdraw? Why all this thrusting?
gpt
It's thought that it's a method to extract rival male's semen. The thrusting and deep penetration allows the penis to essentially scoop out the offending fluids, before he then deposits his own. This is also an explanation for the shape of the head of the penis.