system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do men have a refactory period?

I always wondered why: wouldn’t it be better for the progression of the species  if men could have sex “continuously”? 
Is it to make sure the new sperm will be healthy?
gpt
The human male penis has a big ridge on it at the glans (the head). The reason this is there is that it creates a suction effect for scooping out semen from a rival male.  If there's semen already in there when you're having sex, you'll remove it. You have a refractory period to keep yourself from removing your own semen.