Thou still unravish'd bride of quietness, (line 1)
The innocence of the urn is connected to sexual purity with this line. The point is that the urn is married, but still chaste. The urn exists in the middle between innocence and experience. Its purity is not child-like – it’s a "bride," after all – but it is virginal nonetheless.
She cannot fade, though thou hast not thy bliss, For ever wilt thou love, and she be fair! (lines 19-20)
Just like the urn, the lovers exist smack in the middle of innocence and experience. They have strong sexual desires but haven’t yet acted on them. Humans have always found this state to be especially poignant and attractive. The speaker thinks he has found the way to have the best of both worlds.
And, happy melodist, unwearièd, For ever piping songs for ever new; (lines 23-24)
The musician demonstrates a kind of artistic innocence. He’ll never get sick of his songs, because he’s always playing the same song for the first time. It’s like being able to go back and experience your favorite song again for the first time: over and over.
To what green altar, O mysterious priest, Lead'st thou that heifer lowing at the skies, And all her silken flanks with garlands drest? (lines 32-34)
These lines form another example of how the poem is hinged between innocence and experience. The atmosphere of this scene looks exceptionally innocent: a priest leading a cow dressed in flowers to an altar. But if the scene were to continue, it would show the cow being butchered to death as a sacrifice to the gods, which might lead us to think differently of the priest and the townspeople. As it stands, they seem pious and peaceful.
Of marble men and maidens overwrought, With forest branches and the trodden weed; (lines 42-43)
The innocence of the urn is tarnished in this image. Now, the marble carving of the "men and maidens" looks too ornate and complicated instead of simple and straightforward. It now seems artificial – tainted by the sweat and grit of the artist – rather than natural and effortless. Even the beautiful decorative plants now looks like "weeds" that have been walked upon ("trodden") by countless people.