A. One of the many Mars/Venus issues that women will never come to understand is how men can be friends with guys whom they think are complete bozos. If your buddy is a decent guy she just doesn't click with, explain why he is such an important fixture and point out how he has always been there for you; she'll likely come around if she sees him as your true, loyal friend. Also, promise to talk to him about tapering down whatever behaviors drive her nuts. It will be easier for her to keep an open mind if she knows your buddy is making an effort.
If, on the other hand, he's a guy whom you'd never want your future son to emulate or a man you'd keep away from your sister at all costs, you've got to ask yourself if this is the guy you really want standing by you at your wedding.
Groomsmen are supposed to support your union, not just be fun guys to party with. Your fiancée has your best interests at heart. If the same can't be said for your friend, then your fiancée may have a point about giving him the boot.