I Corinthians 7

12 But to the rest speak I, not the Lord: If any brother hath a wife that believeth not, and she be pleased to dwell with him, let him not put her away.1
Verses 12-16 have to do specifically with infidelity, the possibility of salvation of a mate and sanctification of the family, not sexual immorality. Infidelity in itself does not save, but it does not defile as immorality. There is potential for spiritual healing and oneness with the mate. Sometimes a Christian man may marry a woman that is not born again. Nevertheless, as long as she is chaste (see v. 2) and wants to live with him, he should not send her away nor stop her intention to live with him.
A couple should have a will to live with one another, the male to be like Jesus is to the church and the woman submissive to the husband. So when a woman has that will, the male should take advantage of it and love (Eph. 5:25) his wife. They can embellish their one flesh and psychological-physical union.

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1The Holy Bible: King James Version. (1995). (electronic ed. of the 1769 edition of the 1611 Authorized Version., 1 Co 7:12). Bellingham WA: Logos Research Systems, Inc.