When two people live together without marriage, or just having sex together, this is called fornication. However, when two come together in marriage, this is a Divine binding, and that is what marriage is.
Genesis 2:18-25 is the telling of marriage as a Divine binding. God presented Adam with Eve and established the first marriage.
Genesis 2:24 shows the basis for all marriages from then on. “That is why a man leaves his father and mother and is united to his wife, and they become one flesh.”
When Jesus was asked about divorce, he quotes this scripture, and regards it to a legal, binding relationship of marriage.
See Matthew 19:6 So they are no longer two, but one flesh. Therefore what God has joined together, let no one separate."
When God revealed the law to Moses, there were many regulations regarding marriage.
A man who seduced a virgin and had sexual relations with her before marriage was required to pay the father of the girl and was required to marry her if the father permitted, see Exodus 22:16. Sexual relations with a virgin pledged to another resulted in death by stoning of both, See Deuteronomy 22:23-24. These laws revealed the importance of marriage and the sin of premarital sex.
Malachi 2:14 calls marriage a covenant. It is clear that sexual relationships between a man and a woman should occur only after a legal covenant was established.
All marriages, whether only through the couple’s government or not, establishes oneness from that point on, the lives of both spouses are affected in a profound way, concerning each other, and their children, property, finances, etc.
It is clearly shown that marriage is an act that is part of natural life, as is sexual intercourse. Marriage can be defiled, as can sexual intercourse.
Whether a person is a believer or not, does not define sin, marriage, or a defiled marriage.