The length is thought to be related to sperm competition.

Sperm competition is not an issue because there are no other contenders.

Sperm competition results in a sexual conflict of interest between males and females.

This suggests that sperm competition is important in the mating process.

This appears to be the result of sperm competition.

So is sperm competition a feature of the human mating system, at least in the evolutionary past if not now?

Charles Darwin might have glimpsed the notion of sperm competition, too.