Please 'splain?

Let's say there's a girl (G) who has a boyfriend (B).  And it is the belief of B's friends that certain intimate things go on between G and B.  And it's all kinds of shocking or hilarious or whatever because B has always been shy with girls and the last person anyone would have expected to be getting any of anything.  So then B breaks up with G and she is totally devastated.

What exactly would make B's friends decide that one of their new pastimes will be sexually harassing this girl?  ...