If you're sleeping with the person you're dating, you're telling each other two things. First, you're telling each other that your relationship with God is not your primary commitment. Second, you're telling each other that you are the kind of person that will sleep with someone you're not married to. Do you think that repeating vows to each other will somehow change that? It doesn't. You will enter into a marriage, if you end up marrying the person that you're sleeping with, already telling each other that you will sleep with someone you're not married to. What happens when he goes on business trips, or when she goes on business trips, and things come up? You already know that each of you is not first and foremost committed to God.