Have a conversation with her about how you will approach her having feelings for someone else if it happens again. Don't let her off the hook by her saying that it won't happen again. As far as she can see at the moment I'm sure that she believes with all her heart that she won't ever develop feelings for anyone else during your marriage but since in her heart she believes in open relationships (and by extention having feelings for more than one person at a time) I predict that if it happens 5-10-15 years from now she's likely to keep it from you and have an affair if she thinks that the deal is monogamy or she's out.
Talk to her about coming to you with the slightest incling of an attraction. The smaller steps are far easier to handle than her coming to you and telling you that she's in a full blown love relationship with someone else (even if there hasn't been any physical contact).
If she's willing to let things go with the man she's currently interested at the moment it might be wise to go that way. It's not fair to him to string him along with the possibility that something might develop only to have an outside influence (you) pull it out from under him. Unless of course he's aware of the situation and 100% on board with being a part of the experiment.
Finally if the relationship between you and your fiance doesn't work out it will hurt but that hurt won't last forever. You have learned something very important about yourself and in any future relationship you'll know to bring up open relationships and your feelings towards them before you get too emotionally involved with them.
Everything will be ok in the end. If it's not ok it's not the end.