Ren-ai Shijou Shugi

From ShoujoMagic:

Karate girl Seri tries to kick away her past in order to catch a new guy at a singles party, but Tamaki, a childhood flame from her old karate dojo, shows up to show off and makes her heart pound. Their reintroduction leads to a karate challenge, and if Seri loses, she must become Tamaki's "toy"...!? It's a game of love with more rivals and obstacles (and sex!--LOTS of SEX!) than either Seri or Tamaki could have foreseen!

Includes extra stories:

(Vol.02) Virgin Beast - Dumped for being a virgin, Katsuragi Mio decides to ask Virgin Killer, Tsukahara Masaho for help.

(Vol.03) Scarred Innocence - Naive Moeka wants to fall in love but Kamiya Shun might not be the nice guy she thinks he is.

(Vol.04) A Girl's Mischief: Love's Supreme Principle - Renai Shijou Shugi sidestory.

(Vol.05) Rescued Letter, Complicated Love - Nozomi delivers a love letter to Shigeyama Itsuya from her friend but...

(Vol.07) Ribon

(Vol.08) Flower of Despair, Thorns of Pleasure - Renai Shijou Shugi Sidestory. Takarai Kou's point of view.