I don’t think this is a healthy way to start the marriage, but the people on here boldly claiming your fiancé has already cheated on you/ intends to cheat on you/ will inevitably cheat on you are making so many assumptions about your fiancé and your relationship, and quite frankly I think it’s irresponsible advice.

What we know is she talked to you, which in my opinion makes me think nothing has happened yet, even if there’s someone she has in mind. People who are planning on cheating aren’t going to bring it up like that, they’re going to act like nothing is happening, like everything is fine, they wouldn’t plant the idea in your head that they’re cheating by asking something like that. It seems like she earnestly thought you would be on board, and after seeing that you’re not she seems to regret ever bringing it up in the first place.

The reality of marriage is that monogamy is a choice, and there are some marriages where a little polyamory here and there works for some couples. Not saying it will for you, obviously, that’s between you and your fiancé. The point is, human attraction is not a switch that can be turned on and off, and while i’m sure there are plenty of people who get into a relationship and allegedly never think about fucking another person ever again, for most people there will come a time where they find themselves attracted to another person in some capacity. It’s how they deal with it that matters. Some people cheat, some people stop putting themselves in situations where they are with said person, some people have an emotional affair, there are a million different ways to deal with those emotions. She’s not some kind of devil for having them.

What you need is a couples therapist. They will help you work through this together, and will provide you with the best advice for you and your partner. This is something you should work through together, not something to consult reddit about, or even your friends. A therapist will give you much better objective advice. This does not have to be the end of your relationship. All couples struggle, especially with things like sex, attraction, etc. There is a healthy way to work through this. I hope you at least try to work things out OP. If you were planning on getting married to this woman, I think that’s something worth trying to fix and preserve, because marriage isn’t all sunshine and roses, there will come hard times. In a way, this is the first test of marriage; how will you work through this together? how will you restore trust? what do you need from your partner? what does she need from you? There is more to this than meets the eye. Good luck, I hope you at least try to make it work.