I agree Link, and I think the problems that most people face, is that the original agreement was not detailed enough, so two people enter into an agreement where both have different ideas that they haven't specified.
When one of them wants to alter the original agreement, it should be done with discussion and consent of both parties. Not just one party deciding behind the other's back that the agreement needs to be altered.
I have actually been giving this matter some thought and the following occurred to me:
The sex we share in a loving relationship involves more than just getting our rocks off. Casual, meaningless, one night stands etc, is surely just about getting our rocks off. Therefore if we are not getting enough of the loving and sharing kind of physical attention from our partner then that need isn't going to be filled by someone else (unless we start a meaningful relationship with them, then why stay with the original partner). So if we're not getting enough, and we just need to free frustrations and get our rocks off that is what masturbation is for, doesn't involve someone else, doesn't breach the agreement and is as good as it gets anyway.
Just my ponderings, open for discussion.
Love
swingingstories