You shall not violate the marriage union.
[I chose to go this way, because "adultery" is only one way of doing this.]
We should fear and love God so that we live clean and decent lives in both words and deeds and that each one love and honor their marriage partner.
93. Matthew 5: 27-28. You have learned that it was told the people of old: You shall not commit adultery. but I say to you: whoever looks at a woman and desires her, he has already broken marriage with her in his heart.
94. Hebrews 13:4. Marriage is to be kept in honor by all, and the marriage bed remain undefiled. The Lord will judge the immoral and adulterer.
This is one way we know Jesus is Lord. He knows our hearts. Faith is cradled there and from it all other things flow.