What does Mark 10:7 mean?

"Leave" is from the Greek root word kataleipo. It literally means to depart and not take the object along. When God made Adam and Eve, He told them to "be fruitful and multiply and fill the earth" (Genesis 1:28). After the Flood, when the people joined together to make the Tower of Babel, God confused their languages and "dispersed them over the face of all the earth" (Genesis 11:9). There is a place for tribes and cities and nations, to pool resources and labor for defense, survival, and cultural advancement. But there is also a place for separation.

Some manuscripts do not include "and hold fast to his wife," but it is original to Genesis 2:24 which this verse quotes. The passage in Genesis can mean the man must cling to his wife as he separates from his parents. But it also means he must pursue and overtake her, to woo her and earn her affections. This isn't a passive relationship, but something the man is being told to work for.

In a typical Jewish household, if a son was to marry, he built a room on his father's house and brought his bride to live there. The marriage would have been arranged by the man or his father and the father of the bride, although it's believed she had veto rights. Economically, that arrangement helped the family, but socially, it was challenging. The man worked for his father. The woman provided children for the clan. If the man rejected the woman, she returned to her father. In many cases, there was no opportunity for the man and woman to separate and be their own family. The patriarchal arrangements had great financial benefits for all involved, but it's not easy to "hold fast to [your] wife" when your parents are in the next room.