If the man who is unclean does not cleanse himself, that person shall be cut off from the community, since he has defiled the sanctuary of YHWH. Because the water for impurity has not been sprinkled on him, he is unclean.
#numbers 19:20

It is not a sin to be unclean, but it is a sin to neglect becoming clean if possible. Without a functioning Temple and priesthood, ritual uncleanness isn't as big of a deal, and some kinds of uncleanness can't be removed at all. So we rely solely on the grace of God to accomplish what we can't do for ourselves.