This is one of the more understated themes of this poem. Porphyria seems to be of a higher social class than the speaker. Her reluctance to be with him might have to do with her reluctance to give up social standing. Death, however, acts as a social leveler – killing her makes her social class irrelevant.
The difference in social class between Porphyria and her lover creates a disparity in their power dynamic during the first half of the poem. The speaker murders her in an attempt to bring balance to their relationship.
The difference in social class between Porphyria and her lover makes absolutely no difference: their power dynamic is uneven because of her intense agency and his extreme passivity, not because of her social rank.