God is the author of our sexuality and He defines the boundaries for what is acceptable and what is not. For many reasons that are for our own good, marriage is the only authorized relationship for sexual activity. The marriage covenant between a man and a woman is to be based on the same covenant relationship we have with God.
If we were on trial with God, we would never be good enough. All of us fall short so God changed the ground rules from the Old Covenant that required our performance to earn His acceptance to the New Covenant that offers complete acceptance as a gift with no effort on our part. One relationship is actually probation determined by conditional love, the other is a permanent union for all eternity based on unconditional love. The song “Just As I Am” often sung at Billy Graham crusades carries a lot of spiritual power. If you had to clean up your act and achieve a certain level of achievement before God would love you, it would be utterly hopeless. Instead, God says I Iove you with no strings attached.
Living together is legalistic in the eyes of God and that has never worked. No one ever has gained God’s favor by keeping the law.
Take a look at Romans 3:20. Probation only shows our faults! Please stop here and consider carefully this reality. Absolutely no one is going to enter heaven based on their ability to be good enough for marriage with God. No, the marriage happens first and then God opens the doors of heaven. We in effect say to God, we are committed to you for better or worse, for richer or poorer, in sickness and in health till death do us part, God I will trust in you! God makes the same promise back to us. Together, we enter into the New Covenant on the basis of grace not law.
Apply the same logic to trial relationships (legal) vs no strings attached marriages (grace), God honors only grace marriages. All sexual activity outside of marriage is a sin in the eyes of God (Hebrews 13:4).
Please read this excellent article titled “Fostering Marriage Readiness in a Culture of Marriage Paradoxes” by Jason S. Carroll