I wonder if it would be possible for D to grow into her own kinks and fetishes as she matures sexually into a self-confident woman. Under the tutelage of MC and some of the other influential female characters she might find herself turned on by the idea of seeing the MC with another woman. One option is that the devs could introduce a system of points, like the exhibitionism points, that you can collect to nudge D in that direction. Another is if, as the sexual relationship between MC and D grows, she develops a fascination with seeing the MC get pleasure (she already spies on him as it is) and figures that she could probably get off watching him bang someone.
She's apparently never seen porn or had much exposure to seeing men get off, so maybe if she's exposed to it in the right context and at the right time she'll like it?