Sex Is Zero (2002)

At Sunjong University, a few students are serious, including Eun-hyo, who works hard at aerobics in hopes of doing well in a national competition. Most students goof off: drinking, partying, and focusing on sex. Eun-shik, a law student who's been in the military, is older but socially inept. He likes Eun-hyo; even though his earnest and simple side appeals to her, his juvenile excesses make him irrelevant when Sung-ok, a good-looking rich kid, charms Eun-hyo. They begin an affair, leaving Eun-shik on the sidelines. The national competition approaches and complications arise.

Source: TMDb