MMA is segregated by sexes mostly because people don't like the idea of seeing a man beating up on a woman. In other sports, men tend to be bigger and stronger, but in MMA you're divided into weight classes which limits that advantage.
There are fewer female MMA athletes than there are male, so in training most of the women have no choice but to spar with the men. Rousey does this all the time to prepare for her title defenses, and beats the crap out of the guys who go in against her. Granted sparring isn't quite the same as an actual fight, but I've heard many MMA fighters, both male and female, say that the women could hold their own against the men.