Ok, first of all, did you know most other mammals have penis bones?
Other animals have penis bones! They come in a bunch of sizes, from 8 millimeters for a Bonobo monkey, to 2 feet for a walrus. So what happened to our men?
To give this issue some dignity, lets called the penis bone by its proper name, the baculum (more like frontulum, amirite?). When scientists looked at other mammals' baculums, they found that the length of the bone directly correlated to the duration of intercourse: the longer the bone, the longer the amount of time spent in coitus.
But, like everything in nature, prolonged sexual encounters were not for female pleasure.
Most male animals leave their penises inside the female so his sperm can "work it's magic" before any competition can get in there, but monogamous men don't need to stay inside a woman past the point of orgasm because their mate is probably not about to get boned down by someone else.
Due to our lack of promiscuity, men eventually evolved to not have a baculum at all.