Better The Devil You Know Than The Devil You Don´t Know? US

Seon-joon heads to Yoo-in's house to see him, who would be suffering from a hangover. But when he entered the house, what he encountered was the clothes scattered on the floor and the scene where Yoo-in was having an affair with someone else??
Seon-joon just turns around and weeps on the street, and then bullies approach him... But suddenly, a man rescues Seon-joon and runs away with him. It turns out that he went to the same high school as him and was his first love, Gyu-gun.
Later, Seon-joon finds himself unable to choose between Yoo-in, who asks him to come back to him, saying it was a misunderstanding, and Gyu-gun, who wants to be with him.
What choices do people make in a relationship crisis? The lover who disappointed me or the unknown first love full of romance? Which of these two is my true love?