So the Lord God caused a deep sleep to fall upon the man, and he slept; then he took one of his ribs and closed up its place with flesh. And the rib that the Lord God had taken from the man he made into a woman and brought her to the man. Therefore a man leaves his father and his mother and clings to his wife, and they become one flesh. Genesis 2:24.
You shall not commit adultery. Matthew 19:18; Mark 10:19; Luke 18:20
Whoever divorces his wife and marries another commits adultery against her; and if she divorces her husband and marries another, she commits adultery. Mark 10:11,12;
If a man commits adultery with another man’s wife, both the adulterer and the adulteress shall be put to death. Leviticus 20:10
You have heard that it was said, ‘You shall not commit adultery. But I say to you that everyone who looks at a woman with lust has already committed adultery with her in his heart. Matthew 5:27
Get the picture? Why is God so strict about this?
Once before all time God had a brilliant idea. That was to make two genders of human and to meld them into a unity, similar to His triune godhead. The trick would be how to get them to stay together.
Love is patient, kind, not envious or boastful or arrogant or rude. It does not insist on its own way; it is not irritable or resentful; it does not rejoice in wrongdoing, but rejoices in the truth. It bears all things, hopes all things endures all things. Love never fails.
As good a glue as love can be, it can wear down under gale force conditions that the average human life presents. In another brilliant moment, of which God has many, but this one came once upon a time, in time, God decided to command wedded loyalty.
The King of kings, the Royal I Am, commanded His subjects not only to be loyal to Him but to be loyal to his and her mate. This stroke of genius allows God to determine who would be a good spouse for Himself. This is the rule of the royal loyal.
As long as the one flesh of a husband and wife unit remained loyal to each other, they could be candidates for the happily-ever-after life in Kingdom come, where genders disappear and everyone is married to the Bridegroom Jesus the Christ. Besides in the long run, with loyalty they enjoy a better here-and-now.
This Mark is one more reason why so few people are willing and able to go through the narrow gate.