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.
Megan, my older sister (by two years) was the first of our dad's five kids to quit Catholicism. Our mom had grown up a Methodist but converted to Catholicism when I was in junior high school.
The short story is that one year after she graduated high school she started dating a Lutheran man. She and our dad had some very noisy arguments (which she won a few of) before she told us they would marry in a Lutheran church. Our dad refused to go but the rest of us went.
She and her husband went to Unitarian services and I remember well her happiness when she told me that in a bible study group she had learned that the ancient Greeks were having their Olympic games at the same time as biblical events were happening. She enjoyed Unitarianism.
She and her husband did a few years of serious partying and seemed very happy. About eight years passed and I was in college (after three years in the Navy) when I learned that she had a miscarriage. She had another miscarriage, adopted a girl, and finally had a boy.
I think her being strong enough to argue with our dad--and win--helped her quit Catholicism.
He wasn't an ideological Catholic. He was able to hear our occasional criticisms of Catholicism. He and my sister eventually made up and got along well.