This article says absolutely nothing about why humans lost the penile bone. Ok not exactly true, it intimates that humans (males in particular) copulate for a short period of time so they didn't need it. From which we can infer that human females preferred not to have to spend a tedious amount of time in copulation. :)
That doesn't sound right to me, unless the ancestor species is quite different from humans. The sperm that are likely to get anything done have been shot into the cervical opening. The penis doesn't get into there.