Megan, It is not possible to avoid your family's reaction to your new world view. You can try to be sensitive to their issues, but in the final analysis your family members are responsible for their own feelings. If you try to shoulder the responsibility for their feelings you will drive yourself crazy. Ask them if they think it is possible for a person to believe something he or she does not BELIEVE. It isn't. Is pretense to a belief you no longer hold acceptable? It shouldn't be. Believe me if you are not true to your own desires and those of your fiance you will regret it. After all it is your life not your family's lives.
Just another thought. You didn't present this dilemma. Your family created it by not respecting your's and your fiance's wishes. It amounts to emotional blackmail, and it is unbecoming of your family to expect you to respect their beliefs when they are unwilling to respect your beliefs. This is a truly emotional situation and the only thing you can really do is try to calmly present them with your decision and ask that they respect it. Perhaps if you can do that without getting into the nuts and bolts of your changed belief they can accomodate your wishes.
I agree with Peggy....they cannot live your life, & you can't live their's. Religion is wonderful for the creating guilt, but guilt accomplishes absolutely nothing. You have your wedding day planned as you wish, & nobody but nobody, family or not, should have any control over it. They have to deal with their own feelings & you can't change yourself for someone else......it doesn't ever work......I know, because I tried that many moons ago! All it did was make a big mess......OF ME!!!