Renai Shijou Shugi

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!

(Source: Shoujomagic)

Included one-shots:
Volume 1: Shinju no Kusari Bangai-hen
Volume 2: Virgin Beast
Volume 3: Kakuran Innocence (Scarred Innocence)
Volume 5: Dakkai Letter Sakusou Love (Rescued Letter, Complicated Love)
Volume 7: Ribbon