I know I said this somewhere else (but can't remember where), but I did a speech on this in college.
Like Alexandra said, women are more likely to use a weapon or instrument while men tend to use fists. We can all remember those old cartoons of the big 'ol housfrau chasing the little bitty husand around the house with the frying pan...well, that's not so far off the mark!
Another thing is, a lot of men that are abused often are physically handicapped - and so have no way to defend themselves. Or - as in the case of the men on this board- were brought up "You Don't Hit Girls!!!" That is a very strong inhibition put into boys when they are very young that sticks with them for a long time. (I bet you guys open doors for women, too without even thinking about it.)
And even if a man did get past the "what would people think?" stage; how many people would believe him? Women have a hard time getting police to believe in their abuse, sometimes. It must be twice as hard for a man to convince the authorities of the same thing!
It has been a very silent problem, but thankfully, more men are speaking up about it. Now society needs to start taking it seriously.