What is the ruling on not having children with the first wife when there is a second wife?

This is forbidden because it involves injustice between the spouses, and because procreation is one of the greatest objectives of marriage. It is not permissible to practice coitus interruptus with a wife without her permission, as this entails neglecting her right to procreate. You must be keen on procreation, acting upon the advice of the Prophet, peace and blessings be upon him: "Marry the fertile, affectionate woman, for I shall outnumber the other nations with you."