Nozoki Ana, Velvet Kiss Recommendations
Although possessing quite different aesthetics, the premise of both series involve the establishment of a contract for the male protagonist to sexually "entertain" the female protagonist. It is intercourse in Velvet Kiss, and it is (initially) voyeurism/exhibitionism in Nozoki Ana.
Both, while being borderline Hentai titles, have an actual well-written story behind them rather than a flimsy pretense prose for the sex scenes. Both are also about a male lead character who finds himself controlled through blackmail by the female protagonist, but their relationship, while adversarial at first, grows and changes as the story goes on. I also think both have above average art for this type of genre.
Forced by certain situations, the male lead in these manga reluctantly make a binding contract with the female lead to form some kind of sexual relationship (between the leads). Although there are certainly a lot of 18+ scenarios, the focus of these manga is on character relationship and character growth and not fan-service.
This manga's were both similar to the story. Both the protagonists were controlled and or blackmailed, but not on the bad way. The girl in these said manga's were opposites based on their style of living, but they're the same when it comes to their attitude.
Even though velvet kiss has only 32 chapters, compared to Nozoki ana with 116, every page was packed with feelings. So if you're looking for some good Romance / Ecchi one, I'm sure you'll love this two stories.