What does the Bible say about having sex with a fiance?

Your intention, your heart, is what God 'sees' so to speak. This is what matters. You lying to yourself is the only shield that will protect you from the truth. Whether you have a formal license or not, your heart must remain true. I know plenty of people with formal marriage licenses and ceremonies that cheat, and people still think they are good in their eyes, and the eyes of the Lord. The eyes of the Lord see you heart even if you have glossed over it yourself. So to remain true, you have to always stay in 'connection' with yourself and see your true ententions at all times. This is difficutly because you will have unpure intentions at times, and you will need to SEE it and LET IT go.