When I was a kid, I used to play Mercy against my older sister. Every time she'd beat me, which was always, she'd mock me for losing a battle of strength to a girl. This went on for years, but eventually I started to win and now I'm proud to say I own her at Mercy.
For whatever biological or sociological reason, there's a rivalry between men and women. And, there are innate insecurities among men about competing against and losing to women in athletic competitions, particularly in the combat sports world.
BJJ is no different. But, the reality is, if a woman has the skills and enough power (the primary difference between genders in BJJ), then she's can defeat a man. And, that's exactly what happened in the video below (from No-Gi competition at IBJJF NY Open).
That being said, don't ever let this happen to you, grappler.*
* Kidding. But, seriously, don't.