Nothing about being best man explicitly shows that you approve of the bride. It is a big honor to be asked and you do it with a clear conscience because you are not part of the marriage contract! All you do is safeguard the rings, make sure the other groomsmen are ready, facilitate conversations, help unescorted ladies to their places, and stand there with your friend. Oh and decorate the car.
Speaking from experience, your friendship may be much more durable than your friend's marriage.
