The displacement of previously deposited semen. The increased likelihood of producing an orgasm in the female which improves the odds of conception. The advantage of depositing sperm closer to the objective.
But aren't these related to increases in
erect penis size? Erect penis size has increased along with flaccid penis size, but not very neatly and the variation is greater. While the flaccid penises of most men fall within the 3.5—3.9" range (right near the 'minimum' identified by the study in the OP); the average erect length runs a gamut from 4" upwards of 6".
If it is erect penis size that is being selected for, why don't we see more consistently larger penises? Instead, we see more consistently '
attractive' penises.
I don't see how we can discount sexual selection in explaining the presence of so many 'attractive' penises that are otherwise less effective when erect (according to you) than their larger-when-erect counterparts.
Jon
Love your enemies!