The poet views Emily as his "heart's sister." Bonca claims that Shelley's "self chastisement" follows the "Self-indulgence" of imaginative fantasy and his ideation of Emily as "sister" and a "twin": "Within the poem proper, Shelley suffers 'annihilation' when he engages in the autoerotic experience of making love to a sister-spouce whom he imagines to be his own twin" (121).
The annihilation may be also thought of as a reaction against the strangeness of the experience, the fact that he is in love with an unobtainable woman and that this unobtainabilty has caused him to overtly resist the confines of his mortal existence, including his sexuality.
Bonca, Teddi Chichester. Shelley's Mirrors of Love : Narcissism, Sacrifice, and Sorority. Albany: State U of New York P, 1999.