Robx, I agree that vows are not a blank cheque or a person's property title. The grass is only greener where you water it.
But if you can't commit, why bother getting married? An affair is walking away, leaving marriage by the back door. Even if you are so upset with your marriage you should be upfront.
Hearing someone say she doesn't want to be married anymore is hard but at least is honest. An affair will never be.
People can commit initially, after a few years though of less than favorable conditions, people tend to want to renegotiate contracts for something better, I don't write the rules, this is human nature plain & simple. Again, commitment is just a word, what are the actions behind it?