Seutaga Doegosipeo?

The two of them meet by chance at the theater (to be precise, in the bathroom in the theater), and what starts out as a small misunderstanding turns into something colossal when Sang-Woo mistakes the boyish Sun-Woo for being a man. They take the same train back, and high school idol Sun-Woo proposes a certain 'something' to Sun-Woo in order to escape from attention of other women. Sang-Woo, who is becoming interested in this boy-like girl, accepts her proposal.

(Source: Korean publisher, translated)