The best interpretation of the metaphor comparing the youth of the man and woman to a time “when the wings of the butterfly, though fully grown, are motionless in the sun” is:
It’s a time when youth may be physically grown, but they don’t have the experiences of someone older yet.
This interpretation captures the essence of the metaphor, suggesting that while the young man and woman may appear mature in some ways (like the fully grown wings of a butterfly), they are still in a stage of life where they lack the depth of experiences and maturity that come with age.