Ren-ai shijou shugi manga

Ren-ai Shijou Shugi summary:

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!