A company is merely a place of work for a person, and in essence the company should only have control over things involving that jurisdiction only, giving a company the right to restrict employees from having a family brings their control from out of the workplace, and into the personal lives of a human being.
Family is at the core of daily life for most people. To have companies restrict people from having families would shatter that fact. Many people who have families can work effectively with little to no adverse effects on their work performance. In fact perhaps most people work in this way.