That is how it is normally. There is an old saying that men and women should think about when they consider messing with a person in a relationship with another. "How you got them is how you will lose them."
If they are willing to cheat with you on another or mess with you knowing you are suppose to be committed to another, they are more than likely going to do it to you when you two are together. Essentially, the decision to cheat is a character flaw. Alot easier to just be upfront with your intention to not be exclusive.