Well...that was something.
So what happened was she confessed, he rejected her, she decided to go tits out (literally) as a final act of desperation, and his immediate response was to bolt on her. Later, he's shown to get a boner from her, despite thinking that he can only love Sensei.
Well first, her actions were very questionable, but it's not like I can't see where she's coming from. Shame it didn't do her no favors, however, and at worst she can be seen as committing sexual assault. But I very much doubt the story'll go that direction, and instead will just make everything awkward between them down the line. It can be seen as dumb teenage actions, really.
Second, this proves that despite his convictions towards only being in love with Sensei, the dude's still only 18, and let's be honest, nobody has their whole life and future set in stone by that age, especially when it comes to romance and lust. His mind says Sensei, his body says otherwise. This'll no doubt start making him think about how he actually feels towards her and what he'll want to do.