I think sexual assault by women on men happens way, way more often than is reported but is not as visible for these reasons:
1) Men don’t report it. It makes them look weak. “What do you mean, you let a tiny little girl have her way with you?” is what you’ll get, etc., even if that makes no sense.
2) Women are a lot more subtle about the assaults they commit. They do it when they know they are more likely to get away with it.
3) Relating to point 1, almost no one cares when men do report it. It’s laughed off and not recorded.
Common estimates are that women commit around 2% of actual sexual assaults. I’d bet it is more in the 30% range. Still not as many as men, but a lot. A whole lot.