Waiting until marriage to have sex or to live together (mormons) is incredibly stupid.
I mean, the IDEA and the PRINCIPLES behind waiting until marriage to have sex is alright, if that's your kind of thing, but sex is a pretty significant part of a relationship, and until you've explored your relationship pretty well, getting married is dumb. You might have a perfect relationship until you have sex and if that doesn't work out, most likely the marriage wont either.