Now to the married I command, yet not I but the Lord: A wife is not to depart from her husband. But even if she does depart, let her remain unmarried or be reconciled to her husband. And a husband is not to divorce his wife. 1 Cor 7.
This is the Lord’s commandment.
That’s how I understand it as a Catholic, and teach it to our RCIA students.
it’s actually one of the exceptionless norms, to use a term from Natural Law.