It is generally believed that in dreams, sex represents power.
More than likely, whoever you "raped" is someone upon whom you want to force something on. I would venture to guess you want to force advice on them.
As an example, perhaps dreaming that you are raping your best friend means that in your waking life, you see that she is with a guy you know is bad for her and you want to force her to leave the relationship...but you hold back because you want to let her live your own life.
Well, your mind still wants to let you know your own feelings on the matter, so it might dream you raping her as a way of trying to get you to see that it feels the need to force your thoughts onto hers.
So my guess would be that your dream means that there is someone in your waking life that you feel needs to have something forced on them (most likely for their own good), but you don't want to force it on them for whatever reason.