Ore no Jiman no Kanojo desu.

Natsumi was looking forward to a lovey-dovey summer with her boyfriend. But after finding out he's stuck making up bad test grades all summer, she agrees to go to the beach with her friend Makoto. Little does she know that Makoto is a total slut who only wanted to bring Natsumi to her insatiable fuckbuddies...
(Source: Irodori Comics)