system
Given the following post, respond as if you're commenting on Reddit:
human
Why does sexual intercourse consist of a stimulation through going in and going out (friction?)? Also why do we need to be stimulated in order to reproduce?
gpt
Sexual intercourse isnt exactly friction. In a normal intact male and a normal intact female there is very low friction. The penile sheath rolls over the glans and stimulates both partners in the in and out motion. 

As to why do we need to be stimulated to reproduce, someone else will have to answer that.