Damn straight you should be upset. BUT only if you truly believed you was his best man before he decided to get married. If he has a friend or another family member who he's real close to then you should've had a hint something like that could've happened. Just because you're my best man doesn't mean that I'm your best man. The same rule can be applied for people who are close friends who name the other a godparent. You might name me the godfather of your child but I might elect to name someone else the godfather of my child. The reasons will vary. Maybe you're my boy but I don't think you're responsible enough to do right by my kid if something happened to me where I couldn't care for him or her. Maybe you're a single parent and if the worst were to happen to me, I would like for my kid to be raised in a two parent household. Maybe we were on bad terms at the time.

Times and maturity usually resolves those type of issues and you move on. Either way, it's a sticky situation.