Nana to Kaoru is utterly lame and one-dimensional as it features 20+ volumes without actual sex, romance or even proper SM. The endless teasing after the over-the-top pilot even managed to burn out some of the scanlators and the series is pretty much a hard PG-13 by European standards. The manga´s popularity obviously comes from the female lead Nana, a submissive and a convenient Mary Sue next door, and the professional art, so the manga will probably last to volume 30 without changing in any significant way. My main problems are these:
Why do they have to keep their "relationship" secret? Why is this sexploitative Seinen manga set in a high school? Could Kaoru be more boring? How come that all the sex professionals are the friendliest people possible, who keep hanging out with school kids? How come teens are allowed to work in public sex shops? There are no Shades of Grey on any of the characters due to that. Pun intended and that abortion at least managed to be filled to the brim with sex. Where are the spin-offs coming from? Milking is lame and what is the rest of Kaoru´s harem doing here besides representing (multiple) fetishes? Isn´t this supposed to be a childhood sweethearts romance?
Watch Nymphomaniac and Secretary (2002) instead. Those actually feature developed relationships, Sex/SM and most of all, award-winning acting. The way more erotically charged Natsu no Zenjitsu (9/10) and the actually pornographic Sensei o Mite Kudasai (7/10) both got positive reviews from me, so read them or the Eisner-winning Sex Criminals from Image. Don´t settle for less!
Update: I was dumb enough to flip through the later volumes and even Kaoru´s teacher has joined (a separate) bang bus to add another fetish. I am not able to call the series tame anymore due to the endless graphic sex and blowjobs, thanks to that arc, and the main gang now also partakes in public sex shows while still keeping the swimsuits on. Any more would ruin the Mary Sue factors, a guess? The series ends with 28 + 1 volumes on a bunch of nothing, so my prediction was 97% on the mark.