[DougEiffel]

One hypothesis is that the shape of the glans penis serves to push rival sperm out of the way and behind the glans to be scooped out of the vagina.

I wouldn't call monogamy unnatural. But it may be a more recent invention for humans. We still aren't sure how long it's been the norm.

It's possible that humans used to rape more often. It's also possible that humans practiced forms of polygamy. Either way, cooperative sperm makes sense. It's one of those things where a simple mutation would provide a clear benefit over the competition.