I do and even though it was fun at first it ends up sucking. There was a time before mrT and I were married where we had that kind of understanding. But then again we were pretty much separated but living together to raise our child. There were rules in place like they couldn't be at our house, safe sex for all acts (including oral), and our family came first no matter what. I think you're setting yourself up for jealousy, self doubt, and disappointment. First, its hard to explain to your new fling about their status although some guys are happy just to be freinds who f***. Then when you do come together with your partner you always wonder how you compare. Which means that you don't let yourself go.
I think it's normal to have a roving eye but it's love to refrain from the temptation. mrT and I actually found it more exciting and liberating when we finally decided to get married and love each other exclusively. We also reevaluated our spiritual life at that time and realized we weren't just hurting each other and ourselves emotionally but spiritually as well.
If you still feel like you need some extra spice maybe you and your partner need some space for like a month or two and then you can reevaluate your situation at that time.
Hope this helps!