At the beginning of the poem, the speaker of this monologue is sitting alone in a cottage with a rain storm raging outside. The speaker is doing nothing but listening and apparently he is disturbed as he notes that he sits listening "with heart fit to break." Clearly his relationship with Porphyria is imperfect in some way. Porphyria glides in and adds life to the interior of the cottage. Evidently the speaker had not gotten a fire going; so Porphyria attends to that. She warms the cottage, removes her wet clothes, and calls to the speaker. Porphyria is the active one in the cottage. The speaker just sits there, sullen, and does not respond to her call. The speaker's sullenness and silence indicate something is wrong. Porphyria plays the role of the seducer. One critic's interpretation is that she is a vampire and therefore, the speaker is justified when he eventually kills her. In any case, she is attempting to seduce the speaker in these lines: 

She put my arm about her waist, 

And made her smooth white shoulder bare, 

The speaker remains passive even when she declares her love for him. He says that she is too weak to give herself to him completely because she will not sever "vainer ties" - her love of another man, possible a husband. This indicates why (excluding the vampire theory) the speaker is reluctant to respond to her immediately. However, she does seem to give herself to him fully, at least for this one night. This is when he decides to kill her, insanely thinking that he has immortalized her in the moment when she embraces him fully.