Leviticus 21:15 "Neither shall he profane his seed among his people"

When a Jewish person wants to get married, a special wedding ceremony must be held. Afterwards, the couple is considered man and wife.

The previous Negative Mitzvah (161) cautions a High Priest not to choose a widow as his wife and then marry her.

This Negative Mitzvah warns the High Priest not to live with a widow even without the marriage ceremony.