I think religion is a tricky thing when it comes to weddings. Don't delay this discussion with them.
You might even consider compromising, and having some traditional elements, skippings any religious reference. Have your handfasting combined into part of the ceremony. I don't know if you've heard of offbeatbride.ning.com If you haven't yet,
Sign up! They have lots of ideas on how to smooth this over, even an advice message board for topics like yours. Good luck with your folks.