The "love" between the speaker and Porphyria turns pretty quickly into a power play. Porphyria seems to be the one who's in control at the beginning of the poem, then the speaker completely reverses things. He seems to want to possess Porphyria, so he reduces her to an object (a corpse, instead of an independently-thinking individual).
The speaker of "Porphyria's Lover" murders Porphyria in order to reduce her to an object that can be controlled and admired.
The speaker's power over language breaks down at several key moments in the poem, suggesting that his calm control is only a superficial veneer.