At some weddings the hosts say grace before dinner, and at others they just dive right in and feast. There is no wrong or right way to host a reception. But if you're feeling pressure from your fiancé's family to say grace when you're not religious, here is what you can do to ease that stress.
If you're not comfortable with the situation, let your fiancé know. After all, it's your wedding just as much as it is his. If you don't want something done you shouldn't feel obligated to do it. Let them know that it is more of a neutral reception than a religious one. Your fiancé's family should understand your concerns and you're feelings.
If your fiancé is siding with his family and wants to say grace, make some sort of compromise. You and your fiancé can give a toast or provide a short speech before everyone eats. At the end, your fiancé can say something sweet and simple about blessing the food. This way the event remains unbiased and the two of you — including his family — will be happy.