If a man wins a fight, people say, "Dude, you just beat up a girl".
If a man loses a fight, people say, "Dude, you just got beat up by a GIRL! LOLOLOL"
No one would compete, and if they did, there would likely be public outrage. I'm sure many women competitors would like to see this happen, but no man would do it. Even if he was ok with hitting a woman, he would know that he stands to gain nothing from fighting a woman. If he wins, people will say, "so what." And if he loses, he will be mocked. The public outrage at the UFC holding such an event could devastate the UFC.
The chances of a man on woman fight happening in the UFC are less than 1%