LEV877 The Kohen Gadol must marry a virgin. The main good that a person can have is pure and clean thoughts, for one's deeds follow one's thoughts. Since the Kohen Gadol is the highest attendant of Hashem, it is fitting for him to take a wife who never thought about another man. Her thoughts were about him exclusively--the holiest of the holy. Thereby, when Hashem gives him children through her, they will be pure and fit to serve in the Holy Sanctuary. Our Sages teach that once a girl has reached the age of bogeres (twelve and a half), she is forbidden to become the Kohen Gadol's wife, for by then, her mind might have become set up on a different man. For the same reason, if a girl is betrothed but not yet married and her future husband dies before the wedding, she is forbidden to marry a Kohen Gadol, even if she is not yet a bogeres, for she had set her mind upon a man other than him.