The urn represents an innocent world, unaffected by the suffering and hardship that come with change. The trees never have to deal with losing their leaves, the violent sacrifice of a cow hasn’t been committed yet, and even the urn itself is "unravish’d," or pure. By the end of the poem, however, the speaker begins to wonder if what he took for innocence might actually be a form of cold distance and alienation.
The speaker turns his back on the innocent world of the urn in the final stanza, a move that is not in keeping with the message of the previous stanzas.
Sexual desire is a form of innocence in the poem.