@Doomroar
>you can't back up the idea of NTR being popular
It's not an "idea". Just go to DLsite or DMM and look at their top ranked ero products, a lot of them are NTR works. Also, NTR =/= cuckolding, but that is a can of worms I refuse to open.
>Then why not ero guro? why not gang rape? why not lilicon? why not genderbend? why not yaoi? why not tentacle porn?
Ask that to Ooi Masakazu. We can only speculate what lead to the creation of this series (artistic preference, market demand, editors request, etc.), but ultimately only the author really knows.
>he was doing something quite rare with his works
He's doing the very same with this series. To be completely honest I don't recall that many mainstream manga (read: not porn) tackling these kind of themes, especially coming from an established artist like Ooi Masakazu.
>Compared to his other works, this particular swinger manga is not showing us anything new, it is the start of a college sex club (which granted is just chapter 7, who knows where it goes from here), but so far is quite cliche.
Being a fan of an artist's other works doesn't mean you have to force yourself to read something you clearly don't like.