What does Mark 10:9 mean?

"Joined together" is from the Greek root word syzeugnymi which literally means to harness two people to the same yoke. Once the man has left his parents and held fast to his wife, the two are joined together by God. "Separate" is from the Greek root word chōrizō and means to separate, divide, or depart. Jesus says the man needs to separate from his family of birth (Mark 10:7), not his wife.

Some versions say "…let no man separate," but the "not" gives a broader meaning. "Let no man separate" could mean that individuals such as potential lovers, lawyers, or judges are forbidden from causing or legalizing a divorce. "Let not man separate" challenges cultures, legal systems, and communities to establish policies and engender environments that do not threaten marriage. Still, in the context of Jewish culture at the time, the only "man" who was responsible for the divorce was the husband. Not only was it he who performed the divorce, it was he who allowed himself to become hardened toward his wife (Malachi 2:14).

The wording here sounds like stern legalese, but God has stronger words in Malachi 2:14–16. God rejected the sacrifices of the Israelite men because they sent their wives away. They were faithless to the women they had made a covenant with. God had joined them "with a portion of the Spirit in their union" (Malachi 2:15), but the men chose to stop loving their wives and sent them away instead. God says they covered their garment with violence. Malachi 2:16, which translations often start as "God hates divorce," most literally means God hates when men are faithless to their wives and send them away. These are the fierce, emotional words of God that Jewish leadership taught the men to set aside for their own interests.