Well, if you really love him, like you say, then this attraction to his friend will pass. It would be nothing more than just a 'crush', that is, just an intense infatuation that doesn't last long. I would wait for a while, at least a few months, and see how your feelings evolve. I wouldn't do anything just yet. If, in a few weeks, this hasn't changed, then I'd talk to your partner and tell him you feel like you aren't ready to get married, and ask to postpone the wedding. The worst thing you could do, I think, is to go along and get married, and still have feelings for his friend. Remember, a wedding is a wonderful day and occasion, and it would be horrible to regret that. It would also be horrible because it would hurt your fiance even more. It sounds like you really do care about him, it really does. It sounds like you LOVE him, and so, this attraction with his friend could be nothing more than just that-attraction. I really think it will pass very very soon. Just think about what you'd be gaining by being with him, and think about what you would be losing by losing your fiance. Weigh the two options, but be sure to think long-term. Don't think in the moment, think about where you see yourself in a few years. Good luck.