It's good to be exclusive with someone you're dating.
Timmy has been dating Laura for a few months now.
Timmy wants to see a movie.
Timmy invites Laura on a date with him to the movies.
Timmy invites a female coworker on a date with him to the movies.
Timmy and Laura have a great time and enjoy their time together.
Laura finds out and breaks up with Timmy right away.