John Gill Commentary Leviticus 21:15

"And he shall not profane his seed among his people: for I am Jehovah who sanctifieth him." — Leviticus 21:15 (ASV)

Neither shall he profane his seed among his people
By marrying any such persons, whereby his children, born of them, would lie under disgrace, and be unfit to succeed him in the priesthood, or by marrying among mean persons, or by marrying them to such as were unlawful, and would be a disparagement to them:

for I the Lord do sanctify him ;
separate him from all others, to the high and sacred office of the high priesthood, and am concerned for his honour and holiness; and therefore it became him to observe these laws and rules, and abstain from such disagreeable marriages.