[manspoint]

I just re-read your initial post and was flabbergasted.
"I made the mistake of telling my bf"
You have confessed that you want to have an affair and not tell him. So it was never going to be an "open relationship agreement".
How would you feel if he had an affair? Think of it that way.
You say you are very happy in your relationship...so why then do you feel a need to follow through with this fantasy? What about other fantasies, either now or later? If you decide you'd like to try group sex, swinging, (edit) will you go and do them too?

[Hopeless Dork]

I don't think it being a same sex fling makes it less of a fling. It is what it is. I think you'd be lowering the expectation bar and whenever he feels like it, he could pull the same card on you and you'd be left with only 'yes I did it, but you can't' argument if its not what you'd want for him to do down the road.

If you want an open relationship, go for it... if you are willing to risk him asking for the same freedom to have a fling in the future ... go for it. What may seem like an idea he is 'okay' with at the time can turn into something he can't get out of his mind and chooses to leave you over later.

I think you'd be putting a lot on the line for the benefit of one night of pleasure (or one night of awkwardness and regret)... lets not forget you might not even enjoy it like you fantasize you would. Its a gamble, it puts your relationship in jeopardy for very little gain, in my opinion.

[AliceJW]

I think this is one of those things where you have to make a compromise. if you had a fling with another girl then regardless of whether he knows or not its cheating because you're not with him. If he's willing to let you have your fantasy then I think you should also let him to fulfill his by at least letting him watch since you clearly dont want him participating.
I have an ex boyfriend who wanted a threesome with another girl, I was curious about it as well so we had a threesome. And when I wanted to have a threesome with another guy, although he wasnt really fond of the idea he went along with it and let me have my fantasy too. It's really a give and take situation, and it takes a lot of communication. As the aftermath can sometimes go awry.