"A widow, or a divorced woman, or one who is profaned by harlotry, these he may not take; but rather he is to marry a virgin of his own people, so that he will not profane his offspring among his people; for I am the LORD who sanctifies him." (Leviticus 21:14) The high priest (verse 10) may not take a widow for a wife.
"'A widow, or a divorced woman, or one who is profaned by harlotry, these he may not take; but rather he is to marry a virgin of his own people, so that he will not profane his offspring among his people; for I am the LORD who sanctifies him.'"" (Leviticus 21:15) The literal commandment is that the high priest (verse 10) should not profane his offspring [literally: seed] among his people. Since that would occur through sexual relations with someone G-d has forbidden to him this prohibition is understood that he cannot have sexual relations (even outside of marriage) with a widow.
"They shall not take a woman who is profaned by harlotry, nor shall they take a woman divorced from her husband; for he is holy to his God." (Leviticus 21:7) The priests, the sons of Aaron (verse 1) may not take a woman who has been "profaned by harlotry". The Hebrew phrase translated as "profaned by harlotry" uses a word (tzanah) that implies willful and harlotrous behavior.