Malachi 2:14-15 You ask, "Why?" It is because the LORD is the witness between you and the wife of your youth. You have been unfaithful to her, though she is your partner, the wife of your marriage covenant. Has not the one God made you? You belong to him in body and spirit. And what does the one God seek? Godly offspring. So be on your guard, and do not be unfaithful to the wife of your youth.
Your claim is bogus. There is no such recommendation.
Yes, of course in your head you see it that way.
But then, I see it this way.
Deuteronomy 21:10-17
Marrying a Captive Woman
10 When you go to war against your enemies and
the Lord your God delivers them into your hands and you take captives, 11 if you notice among the captives a beautiful woman and are attracted to her,
you may take her as your wife. 12 Bring her into your home and have her shave her head, trim her nails 13 and put aside the clothes she was wearing when captured. After she has lived in your house and mourned her father and mother for a full month, then you may go to her and be her husband and she shall be your wife.14 If you are not pleased with her, let her go wherever she wishes. You must not sell her or treat her as a slave, since you have dishonored her.
The Right of the Firstborn
15
If a man has two wives, and he loves one but not the other, and both bear him sons but the firstborn is the son of the wife he does not love, 16 when he wills his property to his sons, he must not give the rights of the firstborn to the son of the wife he loves in preference to his actual firstborn, the son of the wife he does not love. 17 He must acknowledge the son of his unloved wife as the firstborn by giving him a double share of all he has. That son is the first sign of his father’s strength. The right of the firstborn belongs to him.