Tsuiteru Kanojo.

is a Japanese manga series written and illustrated by Niku. The manga was serialized on ComicWalker and published in Japan by ASCII Media Works under the Dengeki Comics NEXT imprint from December 2013.

Synopsis
Yoshida Kazuki is your average office worker who is a virgin. On a fateful day during a meeting with his boss, he meets a girl named Futaba, a very cute and lovable girl, and they end up going out together. However she has a big secret: she is a futanari.

(Source: Manga Updates)