Stephen cites an excellent article which describes the why, so I want simply to offer a snippet from it in hopes that you’ll go read the whole thing:
Did you ever notice that there is a different tone and feel to a wedding and the celebration that follows when a couple has been living together prior to taking their vows? Something is missing. Oh, it’s not the guests, the music, the cake, or the decorations. There is always plenty of that to go around. But something is lacking. I will go so far as to say that there is a special look that is absent in the way a co-habitating bride and groom even look at each other. There is no anticipation and no excitement of a new ‘beginning.’ Cohabitation makes a mockery of a sacred vow and sacrament, and leaves the bride and groom without that tangible sense of the life-long vocation and commitment that they are entering into.
Go read the rest. I happen to agree. Sure, cohabitation is common, even among ‘nominal Christians’ (Christians in name only) but that doesn’t make it right. In fact, it just makes it more tragic.