Another theme that develops in this poem is the objectification of women. From the beginning, the speaker reduces Porphyria to her separate (and sexually exciting) body parts: her damp hair, her waist, her bare shoulder. As the speaker becomes more captivated with Porphyria's sexual advances, there is no discourse of her mind or of his unwavering and true love for her. Instead, Porphyria is reduced to an object to be possessed:

That moment she was mine, mine, fair,
Perfectly pure and good

The speaker feels that he has some claim to Porphyria because "Porphyria worshipped [him]," and this surprises him.

He then uses Porphyria's damp hair, which has propelled his sexual desires since she entered the room, to strangle her. Thus, her own sexuality is her demise, both exciting this man and being used to end her life.

In the end, Porphyria is simply an object to be possessed. Even as a corpse, the speaker lays claim to her "smiling rosy little head," a childlike image that reinforces her unequal status to the speaker—both in life and in death.

The Victorian society held beliefs that a woman's sexuality was to be repressed at all times. A woman who ran through the night to meet her lover would have been downright scandalous. Thus, Porphyria is representative of the objectification of women at large who lived within this historical context.

An important theme of this poem is sexual repression. The male who tells this story is a poor cottager receiving a visit from a rich, blond woman above him in social status who wants him as her lover. She makes her sexual desire clear.

The narrator responds to this with desire of his own. Victorian writers had to be circumspect in speaking about sexuality, but clearly we understand the meaning when the speaker says the her worship of him made his "heart swell and still it grew."

The speaker "debates" what he should do. He seems unable to get over his idea that the woman who worships him should be sexually chaste and pure, so he makes her so by killing her:

That moment she was mine, mine, fair,

Perfectly pure and good: I found

A thing to do, and all her hair

In one long yellow string I wound

Three times her little throat around,

And strangled her.

Although there are many themes in each work of literature, if I had to boil this down to one, I would say that Robert Browning stresses the theme of violent love (especially in relation to madness) in his poem "Porphyria's Lover."

Upon first glance, the idea of "violent love" seems like an oxymoron; however, after reading the poem, the relationship becomes clear.  The speaker has a desire to keep Porphyria all his own.  In order to do this, he strangles her with her own hair. 

There is no doubt, however, that love (or at least infatuation) exists between the two:

She put my arm about her waist,
And made her smooth white shoulder bare,
And all her yellow hair displaced,
And, stooping, made my cheek lie there,
And spread, o'er all, her yellow hair,
Murmuring how she loved me

According to this quotation, the love between the two people here is quite passionate and sexual in nature.  Unfortunately, the extreme passion that exists between the two leads the speaker to violence in order to claim Porphyria for his own, forever:

I found
A thing to do, and all her hair
In one long yellow string I wound
Three times her little throat around,
And strangled her.

As love leads to violence, one has to admit that mental illness, or if I may use the more common term "madness," to be involved here.  The speaker talks about how Porphyria's blue eyes "laughed" and how her head was still "smiling" after she was dead.  Further, the act of murder itself caused her to feel "no pain.' The speaker strangles her as he is "quite sure she felt no pain."  He then props her "laughing" eyes open yet again, to kiss Porphyria as she lay there, dead. However, probably the biggest indicator of madness can be found in the last few lines:

And thus we sit together now,
And all night long we have not stirred,
And yet God has not said a word!

The speaker is now sitting, hour upon hour, and caressing a dead body.  Death has not seemed to stop the speaker's sexual attraction to Porphyria.  Further, there seems to be an indication here that the speaker's grave sin would have actually prompted God to speak.  Thus, the speaker sits, clinging to a dead body for sexual gratification as he waits for God to rebuke his action.  Yuck!