I think that, first of all, you need to try to remember that this is your wedding and, presumably, all those people in the audience are there because you’re important to them and all of them know you may stumble with speaking. Secondly, I suppose, just try to check if you can just speak your vows and I-do (im assuming it’s this youre talking about, and not a toast?) without a microphone, just to you, your partner, and the person who will be officiating. Third, it’s your wedding. It’s about you and your partner. He/She/They know and care about you. Hold their hand, take a deep breath, and if marriage is something that is important to you and you’re truly going for it, take the leap of faith. And please, try not to be cruel toward yourself if you stutter, because, it is a big day and there are millions of reasons to be anxious. Try not to punish yourself for any mistake. You will be able to enjoy the reception/honeymoon/life as a couple better that way.