Maybe ask her what difference, in HER life, would you getting married make?
Mummy would still be mummy, dadddy would be daddy, you would all still live in the same house and eat the same food and have the same toys and dog and blah blah... Then explain, that, mostly a wedding would just be a big, expensive party and that you would reather spend the monry on a nice holiday.
Thats what I am planning on telling my DS anyways