Being good is taxing. Being good is ego-depleting, the draining of one’s precious willpower juice. What you give up in one area of your life (I’m going on a diet), might lower your chances of sterling examples in other areas (last night I slapped my kid for misbehaving).
But if we do not have to be good, the question is: what do I do instead of being good?
I don’t think Mary Oliver is encouraging us to be bad, or particularly self-indulgent; I don’t think that’s what she means by letting the soft animal of your body love what it loves.
But neither is this a kind apophatic sermonising (from the Greek: ἀπόφασις apophēmi “to deny”) a roundabout, via-negativa attempt to bring us back, through contradiction and divergence, to the very thing from which we’re trying to get free. As in: “We do not know what Good is. Good itself does not know what it is because it is not anything. Good is not, because it transcends being.” Of course the writer of the previous sentence was referring to God. Same thing perhaps.
You do not have to be good. You do not have to be God, or godly (the knees, the desert, the repentance). You only have to be a human, which in and of itself is tricky.