Having to think about porn while having sex to orgasm is a symptom of porn addiction. I don't see those words, "porn addiction," in your post, and I have no way of knowing whether you are or not, but let me explain how the problem works. We are all initially wired to be fascinated with sex. Love it, hate it, enjoy it, be terrified of it; our brains are wired to be fascinated with thinking about sex. This is especially true for guys hitting puberty. When addicts find high speed internet porn, we have hit the jack pot. I won't bore you with details but HSIP is a mechanism we use that allows us to think about sex constantly, and it never has to be with the same mental image, aka pornographic image or women. Now--that's a problem because we evolved to think of sex with a real woman, and in reality most of us are only going to have sex with a handful of women. Before HSIP the sexual reward pathway that was rewarded was intimacy with a real woman. HSIP though opens up a new sexual reward pathway--now porn can be rewarded, and it can be rewarded over and over and over because it is perceived as endless, new, novel, sexual experiences. When I say "rewarded" I mean with a dopamine high. So, through HSIP, the natural sexual reward pathway of intimacy with a real woman becomes desensitized, and porn becomes highly sensitized. Sounds to me, porn addiction aside, that you have watched sufficient amounts of porn that now only porn does it for you. If in fact that is the case, it is fixable, but it will take some effort. For one thing, you are going to have to relearn how to reward simple human intimacy, and that means during sex, you have to think of the woman you are with, and only her, and allow no pornographic images in your head. In my experience, if you can do this you will find that for about the first six weeks, Oing with your partner will be less satisfying and more difficult than Oing while thinking of pornographic images. But, if you can consistently focus only on your partner during this time, and have no pornographic thoughts during that six week period, then by the end of that period you probably will have re-sensitized your reward pathways to reward actual sex again.
I think this is a choice you need to make. In the midst of my addiction I could not O with my partner, and before I understood what was going on, I finally managed to O, but only while thinking of pornographic images. Once I understood the science behind the problem I was able to retrain my brain to reward good old fashioned sex while thinking only of my partner. Now I only O while thinking of her, only during sex, and never while thinking of porn. Ask yourself, is it fair to you or her that when you O you are not really Oing with her, but with some wacked out pornographic image of her. I think it is time for you to fix this problem.
If you care to read more, my thread.
http://legacy.rebootnation.org/index.php?topic=1256.0
Peace.