Lisa sexually assaults Derek twice.

SPOILERS.

I don't think that Derek knew that an actual crime was committed against him. Female on male sexual assault is not taken seriously, so when it actually happened to this character he just saw it as harassment and not an actual crime.

Derek could have gone straight to the police, it would be a he said / she said case, but it is still worth reporting it to the cops, Lisa sexually assaulted him - the first time was in the men's bathroom stall at the office christmas party, and Derek clearly said the word "stop" numerous times and she did not stop!

The second time was date rape, only it wasn't a date, but it was clearly a date rape drug, she made out with him when he was helpless, he mumbled "please stop" and she didn't. Lisa violated him, it's clearly sexual assault. No one seems to ever mention that when they talk about this film. If it was the other way around it would clearly look like sexual assault and would have been made more of a big deal in the movie and with fans.