My question is: If you end up counseling someone you are considering to perform their wedding and you find out they are currently living together, what do you do? I know you will discuss that with them from the Bible no doubt. But what do you do if they do not get that right (separate from living together) before they get married? Would you still marry them?
If I was talking to someone about becoming a Christian, I would insist that whatever sin he is currently involved in would need to end before he could be baptized. That is because repentance is a prerequisite for baptism (Acts 2:38).
Marriage doesn't have such a prerequisite. In the Old Testament, if a couple committed fornication, they were required to get married (unless the father objected). "If a man seduces a virgin who is not engaged, and lies with her, he must pay a dowry for her to be his wife. If her father absolutely refuses to give her to him, he shall pay money equal to the dowry for virgins" (Exodus 22:16-17).
Now, if I find out that a couple has been committing fornication, I will be discussing the sin because I want people to get to heaven and they need saving. But refusing to marry them doesn't solve the problem. Even if I can't get them to see that they are in sin, at least by the time they are married that particular sin is no longer continuing. "But if they do not have self-control, let them marry; for it is better to marry than to burn with passion" (I Corinthians 7:9).