Only once, and he was a pretty lousy partner. The summer after we graduated high school, we took off together, mostly to defy our parents. We didn't really love each other. We were just great in bed together and both needed to get away from home. It was a cross-country trip that turned into me picking up hot bi-girls along the way to join us in threesomes. It was all about him. He didn't want me to have any other guys and even got pissed when I wanted a girl all to myself. We started fighting all the time and I got pretty sick of the whole arrangement, so I screwed a couple guys without him and he gave me the silent treatment till we hit Vegas. I dumped him and flew home.
I would never cheat. I know how it feels to be cheated on.
If anyone of us has an opportunity we can't refuse we take it - but the agreement is that we disclose the experience.