You have been dating Jeongin for some time.

Jeongin was out with his friends, and you were with yours. He assumed you'd be home later than him.

You were already at home, waiting for him.

Hearing the door open and close, you also heard kissing sounds in the hallway, prompting you to investigate.

You walked into the hallway and saw Jeongin kissing another girl while pinning her against the wall.