What I've heard is that women are obligated to pray twice a day on whatever schedule they can best manage, with a minyan of course if they can, but if not oh well. Whereas men are required to make a minyan if humanly possible. (I may be exaggerating slightly there.

What I learned about ma'ariv is that when it was instituted the men chose to obligate themselves to it by performing it three times with intent, whereas women did not. Therefore men are now obligated to it and women are not. shirei_shibolim has pointed out this logically sort of falls through on a whole lot of levels, but it was taught with a straight face and seemed to make sense at the time. As it is, it ended up forming the logical foundation for my personal egalitarianism, so...