Is Marriage preparation useful, or is it something embarassing the Vicar wants us to take part in?
If the minister is offering marriage prep I'd recommend you take him or her up on it. Gone are the days when the Vicar gave you a stern talk full of metaphor that you just knew alluded to your sex life!
There is lots of good material available now and it's useful to work through some difficult issues now so that you are fully prepared. Even a simple question like "Will you let your mothers have a key to your house?" might well keep you going all night, but I think it's one of the most potent questions. Talk it out now rather than fight it out later