Hit it and quit it

There's speculation that's exactly the reason the under head of the penis is shaped that way.

I read a post some time ago that squirrels have a bone in the penis shaped as a scoop exactly for this reason.

I see you've also read

Semen Displacement as a Sperm Competition Strategy in Humans

Highlight of the summary:

Another intriguing implication of the difference between circumcised and intact males is the question of self-cuckoldry. Put another way, is it possible (short of artificial insemination) for a women to become pregnant by a man she never had sex with? We think the answer is “yes.”