The correct answer is sighing from desire.
Indeed, the lexical field is populated with words that express tenderness, beauty and purity. However, there is a symbolic, underlying carnal desire in the poem. The sibilance is very ambiguous, just as the meaning of the words used to convey it (shade, less, grace, waves, tress). The word “waves” is especially evocative, as it expresses the waves of desire of the narrator for the beautiful woman.