Having your guests stay seated during your wedding ceremony seems like something you would fully expect from your guests, but you would be surprised it isn't always a given especially when it comes to guests taking photos. This seems to be true if someone just got a new camera or is ruthless to getting photos of your wedding day. (To see real life examples I've experience you can read this post.)
One tactful way to make sure this happens is when your officiant welcomes your guests you could ask him to ask them to stay seated during the ceremony.
Let your guests take their photos from their seats. The moment guest start wondering around your ceremony creates a perfect opportunity for your paid professional photographer to miss the moment you most want captured. Trust me, its happened before.