I'm sorry but I so completely disagree.
Regardless of whether or not jewelry or vows have been exchanged, once the terms boyfriend and girlfriend come into play that is a promise of monogamy unless both people have agreed to an open relationship.
Some people never intend on getting married. I was with hubby for 5 years before he proposed. I'm sure if either of us had strayed in that time we wouldn't be married now.
I state again, I hope you are cheated on one day by a girl friend. I hope when you go to propose to a girl she says "sorry, I've been sleeping around and you just aren't my first choice. Bye"