Tatsuo Tokoo explains the relevancy of evidence pointing to the fact that Shelley began composing the poem well before he met Emilia Viviani.
When Shelley met and took an immediate interest in Emilia Viviani in early December 1820, he probably remembered the verse fragments he composed twelve months before stimulated by a woman friend who stirred a similar feeling in him [...]. In the notebook he might have found […] a passage where he advocated theories of Platonic love. When he was inspired to compose Epipsychidion, he transferred the passage to his new poem almost word for word. (100)
I have provided a links to pages from The Poetical Works of Shelley which show "Lines Connected With Epipsychidion" that he had written in his notebook:
Consider, for instance, lines 6-16 from "Lines," found in "Page 1" to lines 149-59 in Epipsychidion.
The second passage is nearly an exact copy! Tokoo uses this as evidence to suggest that the work is deliberately calculated (102). I would not go that far, but it certainly refutes the notion that Emilia is the primary source of Shelley's inspiration. The fact that the poem is at least somewhat calculated suggests that the poem is an articulation of various ideological positions--such as love and monogamy he had been contemplating on throughout his poetic career. The fact that he imported lines, phrases, words, and similar ideas suggests that Shelley was an extremely ideological poet, one who also attempted to wrestle with particulars and coordinate them with his ideological positions.
Shelley, Percy Bysshe. "Lines Connected With Epipsychidion." The Poetical Works of Shelley. Ed. Newell F. Ford. Cambridge ed. Boston: Houghton Mifflin, 1974. 446-48.
Tokoo, Tatsuo. "The Composition of Epipsychidion: Some Manuscript Evidence." Keats-Shelley Journal 42 (1993): 97-103.
