The series has 3 or 4 chapters I think. It starts out with two couples, all students. One couple hasnt had sex and are supposed to be the "pure" characters. The other couple is the experienced couple complete with loose seeming slut. Eventually the "pure" couple try sex and the female just cant get enough. Eventually the two female characters basically switch personalities and there is some NTR. Pure chick just wants sex, even if its not from her boyfriend so the other male lead provides. This has been driving me nuts for weeks.
Image is from Kanyou Shoujo. A tank by the NTR god Nagare Ippon.
Maybe this one?
[Kawasaki Tadataka] Ishi to Kami to Hasami | Rock, Paper, Scissors [English] [SaHa]
In the end they swap boyfriends and pure turns slut, and slut less slutier.
This is in fact what I was looking for. Thanks so much!