Unless a marriage is a God-joined ‘one flesh’ union, the marriage itself is merely fornication (sexual immorality). You must have your own wife or husband…
“But since sexual immorality is occurring, each man should have sexual relations with his own wife, and each woman with her own husband.” 1 Corinthians 7:2 NIV
“Nevertheless, to avoid fornication, let every man have his own wife, and let every woman have her own husband.” 1 Corinthians 7:2 KJV
This means you can’t have a spouse that already is joined by God (one flesh) with someone else.
“Wives, submit yourselves unto your own husbands, as unto the Lord. For the husband is the head of the wife, even as Christ is the head of the church: and he is the saviour of the body. Therefore as the church is subject unto Christ, so let the wives be to their own husbands in every thing. Husbands, love your wives, even as Christ also loved the church, and gave himself for it;” Ephesians 5:22-25
“Wives, submit yourselves unto your own husbands, as it is fit in the Lord. Husbands, love your wives, and be not bitter against them.” Colossians 3:18-19
“Likewise, ye wives, be in subjection to your own husbands; that, if any obey not the word, they also may without the word be won by the conversation of the wives; While they behold your chaste conversation coupled with fear. Whose adorning let it not be that outward adorning of plaiting the hair, and of wearing of gold, or of putting on of apparel; But let it be the hidden man of the heart, in that which is not corruptible, even the ornament of a meek and quiet spirit, which is in the sight of God of great price. For after this manner in the old time the holy women also, who trusted in God, adorned themselves, being in subjection unto their own husbands: Even as Sara obeyed Abraham, calling him lord: whose daughters ye are, as long as ye do well, and are not afraid with any amazement. Likewise, ye husbands, dwell with them according to knowledge, giving honour unto the wife, as unto the weaker vessel, and as being heirs together of the grace of life; that your prayers be not hindered.” 1 Peter 3:1-7 KJV
“And be ye kind one to another, tenderhearted, forgiving one another, even as God for Christ’s sake hath forgiven you.” Ephesians 4:32 KJV