As I mentioned in passing on the Millennial Star marriage statistic thread, my wife and I lived together for four years before we got married. This was one reason I didn't bother going to church during my seven-year dry spell. Not because I felt particularly guilty, but because I knew I was pretty much a persona non grata until I got married.
If I ever go back to church seriously, I suppose I'd have to clear this up with the Bishop. The only problem is that I don't think I did anything horribly wrong. I don't want to be TMI guy here, so let's just say that my wife is and was the only woman for me. Ever. And now we're married, so what's the big deal? The only difference between us and a Temple-attending celestial couple is the date the marriage ceremony was performed.
Don't get me wrong, I don't advocate sex before marriage, but I find it hard to condemn if it occurs in a serious, long-term relationship (with proper precautions). Mormons generally can't have serious, long-term relationships that aren't marriages. I honestly can't say if this is a good thing or a bad thing.
It just seems a little silly to me that unlimited NCMOs are a-ok, but my wife and I are guilty of the 3rd worst sin possible.