The big issue here is the main issue everyone has: they want to be God. People want to make the rules instead of submitting to the God of the universe, so they try to find a way around His plan, but the Bible is clear that sexual relationships are only meant to exist between one man and one woman united in monogamous marriage (Genesis 2:18-24). Ultimately, only bowing the knee to Jesus by trusting the Gospel and turning away from sin to follow Him is the answer. All of us need the Gospel and how it applies to this situation, and that means that since Jesus came to rescue us from our sin so we could live new lives following Him, then we have to obey Him or we aren’t truly following Him. If one partner is seeking to follow Jesus and the other is not, whoever is truly following Jesus should separate from the unbeliever if they are not married and the partner doesn’t wish to follow Jesus (2 Corinthians 6:14-18).
To help share some of what Jesus as Lord would say to a young couple as they live together, we can look at Romans 13:1-7, particularly verses 1-2: “Let every soul be subject to the governing authorities. For there is no authority except from God, and the authorities that exist are appointed by God. Therefore whoever resists the authority resists the ordinance of God, and those who resist will bring judgment on themselves.” This is true even for corrupt governments, as Jesus Himself paid taxes to the Roman Empire during his earthly ministry, and they used some of those tax dollars to crucify Jews and subjugate peoples. Whether a marriage license is just a piece of paper or not, we must submit to the process of obtaining one if we are to obey Jesus as the Lord of the government which is asking us to do so in order to call ourselves “married.” It’s submission to Jesus, which all of us commit to if we say we want to receive Him, be saved, get to heaven, or have eternal life. We can only “have” Jesus and all the life He gives if we have Him as Lord of all.
Ultimately, if we love someone, we will seek the highest good for them, and we’ll show our commitment by sealing it legally in marriage.