You allude to the Kantian view that we should never treat humanity as a means only, but always as an end in itself (see Robert Johnson's explanation in the wonderful Stanford Encyclopedia of Philosophy, http://plato.stanford.edu/entries/kant-moral/#8). Kant didn't think that we should never rely on people as means to our ends at all, but just not at the expense of regarding their humanity as an end in itself. Slavery is presumably a clear case on one side, and on the other might be a trade wherein both parties aim for mutual (not merely selfish) benefit. Whether, and in what cases, a user of pornography might be said to treat the model as an end in herself, is obviously complicated, but these considerations might well be relevant: Is the user aiming at benefitting the model? Is he adequately keeping in mind the effects on the model, on himself, and on the women with whom he will interact, as well as the effects on people generally of a culture in which pornography is prevalent?

The latter questions obviously point in an empirical direction: what are the effects of pornography on its models, on its users, and on others? Is it true that pornography use has psychological consequences that impact how its users tend to think about women, and how they intereact with women? See Caroline West's discussion, again in the Stanford Encyclopedia, http://plato.stanford.edu/entries/pornography-censorship/#5.1 . West's essay also addresses the rather different question of whether censorship laws might be justified.

You write both: "This area belongs to me" and "I am not a bossy person who believes he owns his girlfriend." There's no contradiction if (1) you meant the first as a joke [even if a suspicious one], or if (2) one can own another's genitals yet not own the (whole) person. Kant thought not, but his sexual metaphysics are odd. We might put the problem this way, as a conflict between Roger Scruton (Sexual Desire), a sexual conservative, and Betty Dodson, the guru of female masturbation-as-liberation. Scruton thinks that any woman who plays with herself (be it digitally or mechanically) while with her man (her husband, ideally), commits an obscene display that destroys the unitive meaning of the sexual act--even if (if I read him properly) the woman's engaging in some digitalizing helps them achieve orgasm together or nearly together (unification). Scruton doesn't consider that some men might get turned on watching their companions fool around with themselves down there. Or if he would, the men, too, would be engaged in objectionable obscenity. Maybe you should develop some of this outrageously obscene depravity. (Call me Dr. Phil or Dr. Ruth.) Dodson, by contrast, applauds all forms of female masturbation. A woman must learn to love herself and her genitals (Dodson often says "cunt," which I here mention; but I won't use that word). Further, every woman has the right to sexual self-fulfillment, and may exercise that right no matter what sort of obtuse dude happens to be in her presence or in her life. There's more to be said. Why not try turning the tables? Get out one of those whack-a-jack thingies and sit there during a ball game with an erection for 3 quarters or so. Maybe she'll then better understand what you are feeling when she gets out Mr. Lovejoy from her dresser or night table. Or she won't give a darn--which possibility brings us to another thought. You are merely GF and BF. You are not married. You have made no vows to each other, let alone vows to keep each other supremely sexually happy for the rest of your lives (St. Paul's marriage debt, 1 Cor. 7:2-5). So, as Johnny Carson once said, on TV, to the Governor of Nevada, "lighten up." I think you agree; after all, you ended your question with a smiley face. If and when you and your lady decide to proceed more deeply into a relationship, sexual likes and dislikes should be discussed comprehensively. (Good advice; rarely followed. Some exceptions are, perhaps, Bertrand Russell, Albert Ellis, and Hugh Hefner--womanizers, but at least honest ones.) If your GF's acrobatics with Mr. Lovejoy annoy you that much, go find yourself a lady who appreciates exclusively you and your body parts.