@GoldenWolf
I think it might actually be sexual: we know they have some kind of relationship, they have each others contacts and are using them, even to talk about our MC, but we don't see them interacting much at school: someone in that kind of relationship would try to be subtle in contrast to the MC; so we only know about their relationship because of very specific things a third party wouldn't know. We know he isn't interested in the other girl that likes him and that his research for the club is on female body. Her willingness to do the perverted stuff with the MC, might hint at her doing it before(or more explicit things) as well.
I agree that it doesn't seem he'll do anything with sensei, but I doubt he'll achieve anything with Akari either, he is very blunt about not getting aroused by her what so ever and she'll need to finally accept he is an older women lover. Wasn't senpai interested in her?