I would say direct genital stimulation has to occur, and akane never lost her panties. I wouldn't call dry humping sex, nor grinding. I also wouldn't really define it as sex if Jun just used the brush on Akane, even without panties, if she never touched her anywhere else directly during the session. If Jun kissed Akane or message her breasts while using the brush directly? That's when I would call it sex.
Oh so if your GF came to you and said "I had someone give me a nice rub down but I never took off my panties so since it's not sex it's not cheating right?" How would you react to that statement.
I mean, it doesn't have to be sex to be cheating. You'd have the same reaction if it'd be them having a drawn-out kissing session even without that "nice rub down", and you would certainly not call that sex.