Twenty (2001)

Precocious as a child, Myung-ju discovers sex at an early age. She dreams of a fresh start after she meets Seon-ho. Seon-ho pretends to be an aspiring filmmaker who uses Myung-ju to make a porno film. But when her mother finds them acting naked, Myung-ju run away from home. Dong-woo cannot overcome his inferiority complex over the size of his penis. He is constantly overshadowed by his lover and this only makes his impotence worse. But things change when he meets Myung-ju and regains his confidence. He then dumps his lover and moves in with Myung-ju. But their happiness is short lived when he finds out that Myung-ju is not the pure innocent girl as he thought...