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 emphasizes the idea that while the young man and woman are in a stage of life where they may appear mature, they are still in a transitional phase, lacking the full experiences and readiness that come with adulthood. The image of butterfly wings being fully grown but not yet in motion reflects a sense of potential that has not yet been realized.