First off, let me start out by saying that I am not a new player to the threesome game. I've had a few (two successful, one not so much) and thoroughly enjoyed the experiences- it was, after-all, my ultimate fantasy at one point in time. Here's the issue I'm currently facing: I have known my SO for three years (one of which we have been officially committed to one another). He's currently serving the country overseas and we've had more opportunities to talk about sex more in depth than we did when he was home. The subject of a threesome was brought up in a recent conversation. He absolutely, without a doubt, wants to experience it. While we were talking about it, I was game. I made sure to tell him that with something like this comes rules [The first, and only, bad threesome I've had was a caution-to-the-wind type where I was ambushed with the offer as soon as my third cocktail was consumed. Let's just say, the night ended in tears (not on my part, might I add)] and those rules must be followed in order to ensure the relationship doesn't suffer. He agreed. But now... I'm not so game at the idea. You see, my SO has cheated. He was away for 6 months and let his loneliness- and the impulsiveness that accompanies it- dominate him. Suffice it to say, I almost ended things but I gave him one chance to prove to me that it wouldn't be a continual thing. He hasn't strayed since. But, the thing that has crept into my mind and made itself comfortable there, is that if I agree to this and we find our 3rd and do the deed, is that he'll get a taste for women other then myself. My rational side acknowledges that these insecurities are just that, insecurities; and my rational side knows that he loves me and would do everything in his power to NOT mess up and lose me. But, my emotional side is still worried. Is this is a good idea for me and for us? Or should I take these insecurities and view them as my instincts telling me it's not a good situation and just tell him that, no, I'm sorry, but I won't be able to handle it? At a loss here, any suggestions or insight?