Recently, my family (besides myself) has decided to convert to Mormonism. I am proud of them for finding something they can believe in. However, when they decided to quit smoking they began enforcing (at the recommendation of the Elders that visit us weekly) a "no smoking in the house rule" to help them quit. I understood that it would help them quit and since it was still warm at the time it was no big deal. Now it is several months later and both the people that intended on quitting have long since done so and the ban is still in place. For their belief, I am forced into sacrificing on their behalf. They show no signs of allowing it in the future either.  
 
Now to the point. The wedding is day one. There are still many years of that relationship (hopefully) and in which many events that I am sure will be a family event. Such as Christmas, Thanksgiving, Independence day, ect. They aren't asking for a one time thing. This is informing you that you are going to have to show more toleration than they are going to show you, or tell them to write it out in a nice little letter, fold it up and pop it up their bum. In either situation, one side or the other is going to have to be tolerant to the others way of life. Start reading Romans chapter 14 if you need some guidance on tolerance. One key thing to gather from it in the beginning is that it is okay for you to believe in something but it is not okay to belittle someone for their belief nor should you force your belief onto them for theirs being different than your own.