Sex, or sexual situations, where the girl is on top and clearly in control - no frowning, gaping, blank stares, etc. - usually a grin or lusty expression. Milder than femdom. Basically, she is screwing him instead of the other way around. If the guy is also shown to be enjoying it, it probably fits better in happy sex.
Inspired by comments on post 435995 requesting a tag for "sexually aggressive expression", although "aggressive" implies hostility or violence, which is not the idea here.
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