First off, Charris - you don't have to be on a porn site for porn to pop up. Just something that maybe advertises for a porn site to keep the bills at bay. If you've ever tried to get 'emulated' video games off the internet, you know what I'm talking about. You look for a SuperNintendo video game and wind up with a pair of breasts obscuring the entire screen
Anyway, yeah, you're not really going to understand porn as you aren't a guy - we are INTENSELY visual creatures, and (especially if a guy isn't sexually active with other people) strictly mental fantasies often don't cut it for masturbation.
Women don't get porn, by and large, and I don't really think they ever will. If it really bothers you so much, you can try and ask your boyfriend to leave it behind for you - he just might. But it's tough.
To quote the show 'Coupling,'
"Why on earth would you watch that trash?"
"BECAUSE IT'S GOT NAKED WOMEN IN IT! The four pillars of the heterosexual male psyche are 1) Naked Women, 2) Lesbians, 3) Stockings, 4) and SEAN CONNERY BEST AS JAMES BOND!"
So yeah. Sorry, I got a little caught up there...