Shalom, We learn from the Torah that a Kohen Gadol must be married from the pasuk (verse) וְהִקְרִיב אַהֲרֹן אֶת פַּר הַחַטָּאת אֲשֶׁר לוֹ וְכִפֶּר בַּעֲדוֹ וּבְעַד בֵּיתוֹ. That can be found in the book of Vayikra - Leviticus - Chapter 16/6 (Parashat Acharei) The translation of the verse is: "And Aaron shall bring his sin offering bull, and initiate atonement for himself and for his household." - household is referring to his wife. The Zohar explains that a man that is not married is considered half a man (Or half a body פלגא גופא) making him defective or better said, that he has a blemish. A Cohen that has a blemish (what is called in Hebrew moom מום) can not offer any sacrifices in the temple. We learn this rule from the verse: אִישׁ מִזַּרְעֲךָ לְדֹרֹתָם אֲשֶׁר יִהְיֶה בוֹ מוּם לֹא יִקְרַב לְהַקְרִיב לֶחֶם אֱלֹהָיו -ויקרא כא יז Translation: "Speak to Aaron, saying: Any man among your offspring throughout their generations who has a defect, shall not come near to offer up his God's food." Vayikra - Leviticus - Chapter 21/17 Therefore, in order for the Kohen Gadol to offer sacrifices, he must be married in order not to be invalid or disqualified for the service. You can find much more about this in the Zohar (Parashat Vayikra 5 page 2). As for the kids. The Mitzvah of procreation "Pru Urvu" (“"Be fruitful and multiply and fill the earth” (Genesis 1:28)) is one of the most important commandments. The Kohen Gadol has to atone for the entire nation's sin, therefore he must perform all Mitzvot in the Torah before he is ready to enter the holy of holies on Yom Kippur. Our sages also explain that since the Kohen Gadol must have mercy on all the nation in order to atone for our sin, he must have first kids in order to develop the quality or mercy that a father has for his child. Wishing you great success in your Torah studies. Alon Anava