i know that eowyn's a strong women, mentally/physically... whatever, but that doesn't mean that she could beat Grima, a fairly pathetic example of his race.
in reality, the weakest man can almost always beat the strongest woman (unless they know where to aim
). I'm 5'7", and work outside a lot, with horses etc, so am stronger than average, but a guy quite a bit smaller than me would probably have little difficulty in overpowering me if he put his mind to it.
this has nothing to do with sexism / girl power / pride / strength of mind, whatever.... it's a simple fact of life... guys are stronger. full stop.
on the other hand, i don't know that grima really would rape eowyn. I'd say he was too much of a coward. He'd lose all influence within the kingdom of Rohan if he did, so he probably wouldn't risk.
also, eowyn was the only thing clean or pure within his life, and i don't believe he would let himself dirty something this pure.