One thing I remember in graphic detail from a self defence course that the local police held in our office a few years back.

They were talking about what to do if being raped. If there was no alternative than to submit, for whatever reason, it was suggested that, once the guy had penetrated to try and get your feet inside his ankles and then push out with your legs. It appears that this will cause a sort of 'pulling the wishbone' effect or, as the instructor said "no more babies"!