I honestly only found the answer to this question a few months ago!
I remember talking to a friend once. She was telling me a story about a young guy from overseas who had stayed with them. He told them that when he went back home he was going to live with his girlfriend. My friend said to him, "Has anyone ever showed you from the Bible that it's wrong?" She showed him and as a result he got married instead.
When she told me that, I thought and nearly asked, "How did you show him it was wrong? Where in the Bible does it say that?"