not sure "how to" find people for un-attached sex - I just happened into my situation, I didn't go looking for it.
And... if I learned anything from posting on here about my experience... I'd say don't post on here if and when it happens. I had too many people jumping to use hurtful words and judge me, and I personally don't need it. I know myself and my situation well enough that I don't let what others say negatively affect me, but some people aren't this strong.
You say your have this fantasy, but then turn right around and say that my hiding my BJ will negatively affect my marriage.
So, in your case, is this a fantasy you plan to do something about, or plan to let it eat at you and never do anything about?
I think either way, it would negatively affect a relationship, either by eventual guilt or built up resentment.
I don't have any guilt at this point, but I know that had it gone un-explored, I would have always had questions, and curiosity. Only you can make the determination for your own situation.
It sounds like maybe what you really need or want is some good oral sex, and it doesn't need to be with a woman. Maybe discussing this with your fiancé and expressing how much you enjoy it will get him excited to learn to please you. Speaking for myself, I love to please my wife sexually, and am disappointed if she doesn't have a better time than I do, so maybe he'll be up for the challenge!
Good luck with it, and think real hard before you get into an extra marital sexual relationship. Sex changes everything. I believe that what I’m saying will be very unpopular, but I'm not here to win any contests.