OneStep, you don't beleive in this religion, correct? While it's very loving of your fiance to want to mend your relationship with your parents, that relationship needs to be based in truth and mutual respect. Why are you going to subject yourself to a meeting at the KH that you don't want to be at?
My personal opinion is that you should call your parents ( or whomever is picking you up tomorrow )this evening and tell them you have thought about it and do not wish to attend. Your relationship with God is personal, it's YOUR relationship with Him, and you have some different beliefs than they do. Be respectful, non-judgemental and kind. But be firm that this is a NON-NEGOTIABLE matter. Tell them you love them & will respect their beliefs, but ask that they show you the same courtesy. Don't go into details right now. Don't argue or debate.
You only get one shot at life, honey. Make sure you're the one calling the shots in your own life. I pray that they try to understand. All the best to you on your journey.