מדרש על ויקרא 21:6

6) (Vayikra 21:6) "Holy shall they be to their G d": perforce (i.e., even if they are willing to be unclean) "and they shall not profane the name of their G d; for the fire-offering of the L–rd, the bread of their G d, they offer up — and they shall be holy": This is the cause (i.e., by divesting themselves of the mitzvah of the priesthood, they divest themselves of their holiness.) "they offer up": and not the Levites. "and they shall be holy": to include those with blemishes.