Re: >something to tell the grandchildren
"A wedding is about commitment and while a bit of fun is a good thing, turning the entire concept/event into a joke may indicate the level of seriousness with which they regard it."
I have heard about all sorts of themed weddings, from Alice in Wonderland to Klingon. A wedding is not a marriage, and the fact that they both share the same sense of fun bodes well for the future.
A wedding is about showing your family, your friends, and the world that you wish to be together forever. A marriage is actually staying together forever. I would suggest they have the same chance as any other couple, religious or not, of making this work.
I'll add that I'm not anti-religious. People may believe anything they want, as long as they do not try to force those beliefs on me or restrict my actions because of their beliefs. An agnostic atheist myself, I respect others' beliefs, but I don't think they make them a better person than a non-religious person. What matters is not what you believe, but how you treat others.