Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 162

שלא יבעול כוהן גדול אלמנה, ואפילו בלא קידושים, מפני שמחללה, שנאמר "לא יחלל זרעו, בעמיו" (ויקרא כא,טו); הרי הוא מוזהר שלא לחלל כשרה.

For a High Priest not to have sexual relations with a widow even outside the context of marriage, because by doing so he profanes her, as [Leviticus 21:15] states: "He shall not profane his progeny...." This [also] implies that he must not cause a woman eligible to marry a priest to become ineligible [as happens through the relations described above].