To breach this God-ordained separation between flesh in an unclean state and the holiness of God would be as dangerous to the ancient mind as mixing unstable, explosive chemicals or removing shields from around a nuclear reactor would be to a modern mind. The chemicals and the nuclear reactor are in themselves good, even useful, when properly handled--just like the processes of birth, sex, and death. But a lack of proper separation, a failure to cordon off things meant to be kept separate, could have disastrous results. It was precisely to protect his people and prevent such disasters, and not because natural biological processes are evil, that the God of Israel commanded that his realm of the holy, especially the temple, be kept separate from the realm of human birth, sexual activity, and death. (A Marginal Jew, 4:345.)In other words, God isn't born, He doesn't reproduce, and He doesn't die. Those are fleshly things. He is a holy thing. The two need to be kept separate.
In addition to ritual impurity, Meier also sees categories for moral impurity, genealogical impurity, and impurity as a result of eating the wrong food.