I mean, selective pressure doesn't have to be perfect to influence evolution, and factors that directly influence reproductive success have a high potency. You don't have to know you're getting sloppy seconds to gain an advantage from having a shovel-dick. If there's little-to-no disadvantage, dudes with dicks that clean out vaginas will do better than those that don't.

A close-relative example: gorillas have tiny dicks and relatively weak sperm because there's very little competition for mating. The silverback gets all the chicks and that's that. There's no competition inside the vagina, so they don't have any tools that would help. Humans don't have the same kind of iron trap on sexual pairing, so we have some amount of sexual warfare, so to speak.

Somewhat related, but female chimps make sure to hook up with multiple males, so that none of them can be totally sure if the kid is theirs or not. A male looking to mate with a mother who knew the kid wasn't his is liable to try and kill it, so promiscuity saves lives. When the ladies are looking to hook up with lower-ranking guys, they're a bit sneaky about it too. Presumably the higher ranking guys wouldn't be too happy about that.

Anyway, nature be doing stuff.