This serious tackling of a rejection in a "harem" manga is why it's so much better than other harem manga for the same age range. It takes it and its characters seriously enough to give them satisfying endings to arcs, while also making characters be independent.
Also, I think a big reason Rokudou feels what he does for Ranna is because, whether or not he's noticed it, she's evolved past just having feelings for him because of the mark. She resists his vain attempts to protect her, and stands up to him. She's not just fawning over him, she's growing as a person and almost pushing the limits of the mark.