When my DW and I met, some 23 years ago, we were both enjoying the single life, we were both seeing other people, we weren't exclusive to each other for the first 12 or 18 months. Gradually, we became exclusive and it was great. Then I made a mistake, got involved with someone else and our relationship nearly broke up (previously mentioned on MN), but we toughed it out and we were back to being exclusive.
Then one day, my DW also strayed, and confessed it to me pretty promptly thereafter. Was she trying to even the score? Maybe, but from that event, we recognised that whilst we love each other deeply, sometimes stuff happens and it needn't be the end of a close, loving and long term relationship.
So from then on, we didn't necessarily go out hunting for extra marital activities, but if something special was on offer on rare occasions, we knew we could play - with the others consent. We even swung together quite a few times (after being invited into a 4sum on holiday and loving the experience) and all was good and we have recently started checking the scene out again and been considering revisiting this activity. These days we are pretty strictly exclusive, certainly on my side, although I had my suspicions once or twice in recent years. In fact, there might even be something going on for her right now and while I'm not over the moon with it, I'm not packing her bags either. I know it won't last.
The main rules we have are - don't rub it in the others face (so to speak) be discrete, don't do it with mutual friends, never in our house and never, ever, put the other person ahead of the family.
I'm awaiting my flaming for daring to stray from the rules of married normality....