Numers 36

The heads of the people of Manasseh are concerned that if a woman marries into another clan, that clan will then inherit the land going forward, and their people's inheritance from God won't be upheld for things like the year of Jubilee.

To understand this more fully, go back and read Numbers 27, and read more about what God said about a man who had no sons, how to keep the inheritance in that family instead of a man's name and inheritance being forgotten.

The important thing today, not being under the Law and not needing to worry about family inheritance and the year of Jubilee, is to be evenly yoked. Marry someone who loves God, who has the fruit of the Spirit, and has similar yearnings for a family (or no kids at all).