I think, broadly speaking OP, you need to learn to deal with it. If it bothers you that much then ask him to be more discreet about it.
Rightly or wrongly, men nowadays watch porn. The internet has made it so easy to obtain and conceal that they simply won't ignore it. It clearly bothers you that your boyfriend watches it, but he won't stop, and if you break up with him because you can't handle it you'll have a nasty surprise when every subsequent man you are with also watches porn.
As much as you claim that the porn is replacing you (he's fantasising about other women) he could claim that the toy is replacing him (you're getting physical stimulation and orgasms from it). Now, I'm sure you can say that the toy is providing something different to sex with him, but as a man his brain is wired to think that physical stimulation is the most important thing about sex - and so something that does it better than him is going to replace him. He finds it much harder to separate the pleasure of the toy and the intimacy of sex.
In the same way, you're struggling to separate the images of a woman with the desire to have sex with her (and thus not with you). A man's brain is wired differently; he's not fantasizing about that woman in particular, he's merely enjoying the sight of a sexual encounter. It's firing off all the appropriate parts of his brain which will lead to the result he wants - the orgasm. As far as I am aware, women tend to spin out their mastubatory sessions. They enjoy the feeling and the build-up, will tease things out and enjoy the whole thing until finally they get their orgasm. A man wants to have his orgasm as soon as he possibly can, and he doesn't give two ****s about the build-up. Porn just speeds that along by getting everything revved up in the first place.