The speaker warns us not to treat beauty as a moral thing. Beauty is not “good” in any safe, gentle sense—it is more like a wild gull than a tame dove. Calling it wicked is equally destructive, because the word burns it up. The real danger, though, is loving beauty too much; that can be even worse. Beauty is neither good nor bad, but innocent and wild, with the hard, unsoftened heart of a child. If you try to enshrine it, to keep it safe or make it holy, it dies. The mood is delicate and a little ominous, mixing awe with caution. The speaker feels protective of beauty’s untamed nature and worried about our impulse to control or adore it into something it is not.
Beauty — Summary
Say not of Beauty she is good, Or aught but beautiful, Or sleek to doves' wings of the wood Her wild wings of a gull.
Call her not wicked; that word's touch Consumes her like a curse; But love her not too much, too much, For that is even worse.
O, she is neither good nor bad, But innocent and wild! Enshrine her and she dies, who had The hard heart of a child.