Yeah, I can believe that 1 in 4 has been sexually assaulted. That term encompasses pretty much all unwanted touching of certain areas... like when some idiot grabs a stranger girls ass, out of nowhere, at the bar.
I don't go out to clubs very much anymore, but when I did, I saw that happen a handful of times. They always got booted, but it still happened. I'm reasonably certain that I could ask almost any girl I know, attractive or not, and they'd have gone through something similar. Not all sexual assault is violent, as in physically harmful, but it does include groping and what not.