Skirt no Naka wa Kedamono Deshita.

Kominami Shizuka is a mild-mannered college student who is having trouble adjusting to city life. One day, she meets Kirishima Ryou, another student who projects the image of a cool older sister. The two hit it off, but Shizuka is shocked when Ryou attempts to seduce her. Shizuka initially assumes that Ryou is a lesbian, but Ryou is in fact a man who dresses like a lady.

(Source: Crunchyroll)

Note: The anime aired in three versions: an all-ages broadcast version, a 15+ version on AT-X, and an uncut 18+ version streamed on ComicFesta Anime Zone.