Okay. To clear up some stuff for any future people reading this chapter that have questions.
Part of this series(ch. 1-7) happens before Black Label.
The rest happens after (ch. 8+).
They aren't actually dating. They both genuinely love each other. They both even KNOW that they love each other. But Kaoru is incredibly scared of the implications of him dating Nana. He sees Nana as way too high above him. He thinks he doesn't deserve her and that by dating her, he would socially drag her down, hurt her reputation, and reduce her opportunities. That's why his big character arc is making it into a prestigious university. He wants to be someone who wouldn't drag her down. I don't really know why Nana hasn't confessed to Kaoru, though. For most of the series it was because Nana was scared she wasn't ACTUALLY in love and was just feeling like this because of her being his sub. But by this point, she obviously does. I think it's more about her being scared Kaoru doesn't love her back. Idk, his motivations are much more clear cut than hers right now.
Either way. They aren't 'really' dating. They have a mutual love, are doing their secret BDSM relationship, even know the other loves them back (Nana even basically said she wants to stay with Kaoru and doesn't want to leave him), but Kaoru is still in his character arc of fighting for the feeling of being worthy of Nana and doesn't want to make anything public because of that. That's why they were only able to show their true feelings in Black Label, away from society.