Eh, they had co-ed showers in the dorms way back when I was in college, too. But that just meant people would wear something before leaving the shower itself. Unlike the MC, we were smart enough to keep the towel within arm's reach of the curtain.
As I said, I can kind of see Jill wanting to shower simultaneously if she thinks the MC is a decent sort of guy. It saves time for the rest of their date, and if the MC tries to take advantage of the opportunity then she'll know not to trust him.
It's just that if we
do make the MC fondle her (though
Manuka is right, grope is probably a better term), Jill reacts like a concussed duck instead of, well, Jill.