Being turned on by something that could ruin my relationship.

Well, where to start! Well my fiancé and I have been together for 4 years now. And within these 4 years, we have had amazing sex. No questions asked. However, we went to strip club a couple of times with friends, and I loved the attention he was getting from the strippers, or when other ladies would hit on him in front of me. Then within a couple days of that, I would touch myself imaging him with another woman. Months after that, he would tell me about how he would Fuck his last girlfriends and I was extremely turned on, I would touch myself thinking about it, when we would have sex I would think about it. Made me feel dirty, but so accepting of the fact I love it when he gets attention. I want to take things to the next level, like a 3 sum or him have a girlriend, short of a poly relationship. However I am worried about how he may feel about this. I am not worried about how I feel, I have already accepted it and I have no self esteem issues. However, I am worried that he may not feel good enough, those kind of feelings. So, what is my next step? And if things go well and we approach this whole situation and decide to go forth, what do I do? Any suggestions or stories that could possibly lend insight?