A pretty normal assumption for Sakura, what she's seemingly determined to do now is prove her "values" aren't wrong, so if accepting that offer falls under that then sure.
As for Fumi, it's a bit more complicated then her just saying no, she already agreed but Shinobu is the one hesitant to go ahead with it so he's the one stalling out and not Fumi. They can't just not do the scene because the movie needs appeal, it's not about rewriting the script to suddenly not have a sex scene included.
With this in mind both sub plots still don't exactly intersect to have Shinobu Sakura and Fumi somehow meet or get involved, so something else will have to happen.