Similar anti-mongamous statements can be found in Shelley's "Against Legal Marriage":
But if happiness be the object of morality, of all human unions and disunions; if the worthiness of every action is to be estimated by the quantity of pleasurable sensation it is calculated to produce, then the connection of the sexes is so long sacred as it contributes to the comfort of the parties, and is naturally dissolved when its evils are greater than its benefits. There is nothing immoral in this separation. Constancy has nothing virtuous in itself, independently of the pleasure it confers, and partakes of the temporizing spirit of vice in proportion as it endures tamely moral defects of magnitude in the object of of its indiscreet choice. Love is free; to promise for ever to love the same woman is not less absurd than to promise to believe the same creed; such a vow in both cases excludes us from all enquiry. The language of the votarist is this: the woman I now love may be infinitely inferior to many others; the creed I now profess may be a mass of errors and absurdities; but I exclude myself from all future information as to the amiability of the one and the truth of the other, resolving blindly and ins spite of conviction to adhere to them: Is this the language of delicacy and reason? Is love of such a frigid heart of more worth than its belief? (46).
It may be interesting to think about the possible connection between Shelley's conception of love and the poetic process: one of trial and error. Certainly, the uneven breaks in rhapsody in Epipsychidion show the ebb and flow of commitment and hesitancy. The apostrophic naming of Emily in the early lines--"High, spirit-winged Heart!" (13), "Seraph of Heaven!" (21), "Thou Moon beyond the clouds!" (26), etc.--suggest Shelley's love of the idea of love rather than the actual love of the person.
Shelley, Percy Bysshe. "Against Legal Marriage." Shelley on Love. Ed. Richard Holmes. Berkeley: U of California P, 1979. 45-48.