Well as an example, the scene in the locker room where you choose between Maya and Kara. Currently if girl dicks is enabled and you choose Kara, Jace will question you and you'll say something like "that's even better". If you have it disabled, you seem to have second thoughts. De-prioritized (or however you want to phrase it) would have you respond more along the lines of "Hmm, not really my thing, but I don't mind. She looks good regardless."
It would be for people that want to go in a xenophilic direction, what with Hailthrons and their horns and tail, but also play the protagonist as being accepting of other races and their anatomy, rather than just their cultures.
Another example could be if you have voyeur enabled and feet de-prioritized. If there's a voyeur scene where the people involved are doing feet stuff it would still play because that's them doing their thing. The protagonist isn't engaging in it, so it's a non-issue one way or another.
I hope that made sense and my explanation wasn't too bizarre. Let me know if you need me to clarify further.