Sforno on Leviticus 21:7/ספורנו על ויקרא כ״א:ז׳

1כי קדוש הוא לאלוקיו, every member of the tribe is by definition בעל בעמיו, an especially distinguished personage among his people. If he were to marry someone guilty of or unfortunate enough to belong to the categories mentioned in this verse, the husband would no longer be entitled to the sanctity his birthright had not only entitled him to but had expected him to preserve.