My boyfriend and I, early in our relationship, each established our own List. The List refers to an assembling of celebrities chosen by your partner that you permit him/her to sleep with should the situation arise. The key is keeping the list short. Is it okay if your significant other hooks up with some reality TV slut at a party? No. But if they've been fantasizing about Jessica Biel for years, it's really only fair to let them have her if possible. Plus, that means you could shag your celeb of choice! (Retroactive additions to the list cannot be permitted.)
Please do not send me hate mail accusing me of being a homewrecker. I am in no way endorsing the active pursuit of the stars in question, because obviously no one should be doing that in a relationship. This is only if said celebrities fling themselves at you.
I think 3-5 celebrities is a fair number for your List. Mine includes Paul Rudd, Patrick Wilson, Simon Pegg, James McAvoy and Tony Leung. My boyfriend's consists of Scarlett Johansson, Tina Fey, Kristen Bell, and sometimes Kirsten Dunst ("drugs have not been good to her," he says). I conceded that Scarlett Johansson is so ridiculously attractive that if I found the two of them in a compromising position I would probably hop right in. I'm completely straight, but this line of reasoning provides a bonus for couples of a gay or partially gay persuasion: List overlap.
So, fair readers, I strongly recommend that you have this conversation with your loved one today - because somewhere down the line, it could save your relationship. One a more practical level, it proves whether or not they have the sense of humor necessary to engage in completely serious conversations about absurd hypothetical situations and can do so without freakish jealousy and possessiveness - because if not, you should probably break up with them anyway.