Dear Suzie,
I've had a friend with benefits for the last few months, but we'd made an agreement that we wouldn't sleep with anyone else, or that we'd tell each other if that happened. We were seeing each other around a couple times a week and the other day I was in touch to see if s/he wanted me to stop by, and s/he told me s/he'd slept with someone else. It had been a while and s/he hadn't mentioned it until I was in touch. I feel hurt. Should I confront him/her about it or should I let him/her go?

Signed,
Some Strings Attached

Dear Some Strings,
A FWB agreement, like any other agreement between two parties, is one that has to be based on mutual respect and trust. If it isn’t, it will fail. This is why it’s perfectly normal that you feel betrayed by this person: you had an agreement, and they broke it without any regard for your feelings.
Your FWB must be confronted about his/her behaviour. Don’t get upset when you bring it up with him/her. Just ask them, in a straightforward manner, why they didn’t let you know about having slept with someone else. Then move on. If you do decide to do this again, keep it fun, but insist on ground rules that will keep both of you happy. There are plenty of other hot, respectful people out there who would honour the rules of the FWB arrangement. You don’t have to stick with the one who, really, has not been much of a friend to you.

Sincerely, Suzie