Koi wa Kataru ni, Ochiru Mono.

Socially inept college student Natsume Ise falls in love at first sight with the cute and popular Haru Tsuda. Natsume's friend catches him staring obsessively at Haru and warns him about his promiscuous ways. Natsume doesn't want to believe it, but Haru comes to his apartment and asks to stay over after being caught cheating with two other girls. Natsume pounces on Haru without thinking, and Haru lets Natsume have his way with him. The two become friends with benefits, but Natsume wants to be more than that. Natsume confronts Haru about it, but... This is a story about how an innocent and inexperienced hottie and a promiscuous cutie go from friends with benefits to boyfriends!

(Source: Renta!)