The issue with that is that the sex as dominant conversation option is meant for whenever a character/player wants to be the top. There's the possibility of scenes with shared lead where some characters have their lead disabled (effectively making it a scene with some characters acting as top and others acting as bottom), but I don't remember what the exact requirements to get them to trigger were.
Viewing post in Animations progress report and medium term goals comments
I think there was that scene with Val and Mir after breakfast where you and Val would share the lead with Mir being locked in as a bottom if you decided to side with Val. Don't really remember any non-story scenes where it happened though.
I mean I get why it works the way it does, it was just something that I noticed - in part because the story scenes introduced something like that. And after several instances of being immobilised while the top was busy with someone else, I couldn't help but wonder how to get a bit more variety