God sanctifies a Christian marriage through the righteousness of Jesus Christ bestowed upon the husband or wife through their belief.
“For the unbelieving husband is sanctified through his wife, and the unbelieving wife is sanctified through her believing husband; for otherwise your children would be unclean, but now they are holy.” – 1 Corinthians 7:14
We can be judged according to man’s interpretation of righteousness, or we can be judged based upon God’s mercy through Jesus Christ as it relates to marriage.
All human flesh is filthy because it is constantly enduring the dying process. We have to maintain proper hygiene on a daily basis just to stay healthy.
If you partake in any kind of sexual relationship with a person you are not married to you will be looked upon by the Lord as putting on the filth of the flesh.
He will judge your actions as unrighteous accordingly.
However, God justifies all physical relationships inside of a Christian marriage as being righteous and undefiled because a believer has put off the flesh and put on Christ.
“Marriage is to be held in honor among all, and the marriage bed kept undefiled; for fornicators and adulterers God will judge.” -Hebrews 13:4
Your Christian marriage is a holy institution in the sight of God and one He takes very seriously.