Good questions.

Are there mistresses they say "no" to vaginal sex? Yes, there are. Some of us prefer "outercourse" activities and we also have to take into consideration the desires and needs of our submissives. Not all of them want to have intercourse.

What if they do have intercourse?
Then a "guy goes in his femdom mistress" anyway she wants. (Disclaimer" following their pre-negotiated rules, limits, etc. It's up to the couple, not dependent on the role.)

It doesn't matter who's physically on top or bottom, if they
are having sex as part of their playtime, what matters is who's in the lead role.

The same can be said for a male dom and female sub.
If a female sub is riding her master like a wild stallion, that doesn't mean she's in charge or that he's given up control.

Getting back to the femdom issue. Let's say a femdom likes to suck and tease her male subby's cock. Does that make her less in charge? I say not.

In this case, it's all about the power exchange, the thrill of the game and the roles taken for the play time.

As a side note, your right. We don't have a lot of femdoms on this forum discussing vaginal sex with their male submissives. Don't let that fool you into thinking it doesn't happen.