It's difficult to build and write a consistent route if the player has the option of skipping all the LI's sex scenes though, since every scene between them would have to account for any possible combination of previous scenes being skipped. If any sex scene is skippable, than all sex scenes become superfluous to the story, which beats the point of a porn game.
If future content on the Fiona path is built around the idea that she and Ashe have had sex, and Fiona isn't in the position yet that she'd be comfortable being the receiver, then of course it might be mandatory for her route to include a scene where Ashe gets fucked by her. Hell, in this episode her character development focuses on her concerns about how her and Ashe's relationship will change due to them becoming sexually intimate.