Faithfulness in a marriage is the only way to stay as one flesh.
“For this cause shall a man leave his father and mother, and shall be joined unto his wife, and they two shall be one flesh.” — Ephesians 5:31 (KJV)
“Marriage is honourable in all, and the bed undefiled.” — Hebrews 13:4 (KJV)
Faithfulness produces purity in a marriage.
“Let the husband render unto the wife due benevolence: and likewise also the wife unto the husband.” — I Corinthians 7:3 (KJV)
“Art thou bound unto a wife? seek not to be loosed.” — I Corinthians 7:27 (KJV)