To my mind, a man watching pornography, as well as a woman dressing immodestly, are symmetrical sins.
The temptation to expose is more characteristic of women; the temptation to watch is more characteristic of men. A woman will react with depression, anger, and disgust when she discovers that her husband (or other man) is watching porn; a man will react with disgust, anger, and depression when he is forced into close quarters with a woman dressed immodestly (e.g. when he is in church to pray, and a woman dressed immodestly walks in and sits in front of him).
Both are sins against purity, but not the same thing as actually engaging in sexual intercourse with a member of the opposite sex. However, they can pave the way to the next step in the downfall from purity, i.e. actual sexual intercourse (fornication, adultery).
When a woman walks into a room (or into a church building) wearing a short skirt, deep cleavage, etc, she gets a kick out of the reactions she provokes, she does it in order to tempt, ensnare, humiliate, and wield power over other people. And that’s why men (at whom the snare and power play is directed) will react with disgust, anger, and depression. At this point, she did not engage in sexual intercourse with anyone, but she might actually succeed in arousing inordinate passions in someone, and follow up the opening temptation with flirting and sexual intercourse. Let’s just remember the current epidemy of 20-something and 30-something female school teachers preying on and actually having sexual intercourse with 13- and 14-year old school boys. Those cases start with the typical female sin, dressing to tempt, followed up with flirtation, ensnaring the young boy who is ill equipped to recognize the snare, and finally wielding power over the ensnared boy by having sexual intercourse with him and collecting him as a trophy.
With men, there are also various levels of sinning against purity, starting with watching pornographic material in a non-interactive manner in the privacy of one’s home. Some men will go further, interacting with lewd women through internet chat rooms and by visiting strip-tease clubs. Then, a certain number of men will go even further than that, and will actually hook up with women from adult internet chat clubs and strip-tease bars, and will have actual sexual intercourse with these women.
I would still differentiate though a woman who merely dresses immodestly from one who engages in sexual intercourse with middle school boys, and I would differentiate a man who merely looks at pornographic images in a magazine (or computer screen) from one who hooks up with women and has sexual intercourse with them.