He is not to defile himself for those related to him by marriage, and so profane himself.
It was before appointed that the priests should teach the people the statutes God had given concerning the difference between clean and unclean, Lev 10:10, Lev 10:11. Now here it is provided that they should themselves observe what they were to teach the people. Note, Those whose office it is to instruct must do it by example as well as precept, Ti1 4:12.
Commenting on Leviticus 21:1-9
But he shall not defile himself, being a chief man among his people,.... Which is not to be understood of any lord or nobleman or any chief ruler or governor of the people; for the context speaks only of priests, and not of other personages; besides, such might defile themselves, or mourn for their dead, as Abraham did for Sarah; nor of any husband for...
But he shall not defile himself--"for any other," as the sense may be fully expressed. "The priest, in discharging his sacred functions, might well be regarded as a chief man among his people, and by these defilements might be said to profane himself" [BISHOP PATRICK].