ibdoer, you are worrying too much about it, which will makes things worse. You are the guy...sit back and let your fiance do her part as a woman and plan the big day. =) Guys don't normally have to do much, so it shouldn't be nearly as stressful on you as it was for any woman with Crohn's who got married (I did the planning and got married without any problems related to my Crohn's). Just make sure that the menu you choose is Crohn's friendly (or don't eat anything that you shouldn't). I've heard of people on this forum (or maybe it was the arthritis forum) whose arthritis were flaring and couldn't walk down the aisle without assistance on their big day, but they didn't let that ruin it. Just be happy that you are able to get married and that you have found someone who is willing to live with your Crohn's, as it doesn't always make things easy. BTW, congrats!
The day of my wedding, I had the same fears. So in the morning I took some Immodium. I did go some before the ceremony, but once I went down the isle, I don't think I went again all day. I just put it completely out of my mind and enjoyed myself.
Best advice...don't worry about it. Just enjoy yourself. And Congrats.