Just remember that the Kouhai just forced the senpai into the hotel in the first place. Funny how someone who think the other is the worst consent to a relation. The author sure didn't bother to explain how it worked. Magic i suposs. Can't appreciate it when I know how it begin.
Given that Tougou describes their relationship as "friends with benefits", apparently the sex is so amazing that she's willing to put up with Rika's bad traits.