This is one of the posts intended for the ladies. :D
Not true. The definition of "sit," from Dictionary dot com: "Verb: 1. to rest with the body supported by the buttocks or thighs; be seated." In the picture, the woman's upper body was resting on the guy's chest, her buttocks wasn't on his face.
What do you do when someone invites you to take a seat? Do you lay your belly on the chair, and hang over it? Do you rest your crotch on it, 69 style, and lead forward so that your "weight" is on your crotch? No! You plant your butt on the chair, you sit! :D
So what do you do when a guy asks you to sit on his face? Something very similar to what you'd do if you were invited to "take a seat" of course. :D You'd smother his face with your buttocks, you'd rest your weight on your buttocks; hence on his face.