Well, I'm sure it can. But as with everything, moderation is the key. If you condition yourself to only get off to certain things, it's possible that you'll find it harder to reach orgasm without those things. There's nothing wrong with a little fantasy here and there, so that in and of itself shouldn't be an issue. But if you're enjoying yourself more than your girlfriend, you might just have a problem. Is there some way you could incorporate your gf into your erotica fantasies, if she's interested in playing along? If that's not an option, you could try kicking the habit for a little while so you can be more easily aroused by regular sex.

I think that it is perfectly normal. 99% percent of all men I have ever known masturbate to porn. Most that I know like videos or magazines, but I like it that you also like erotic literature, much more sophisticated and creative. Have you ever considered reading the stories out loud to your gf? She might enjoy this and it could be good foreplay. You could let her pick the story, or pick one that you think she might get aroused by. I don't think that porn can negatively affect your sex life unless you start to think that that is how women really are. Most women do not have bodies like porn stars. Women in the real world need foreplay. Lots of foreplay, like at least 20 minutes of gentle clitoral stimulation in order to orgasm. Also, women need men to treat them affectionately and intimately, not the way that women are treated in porn movies. Most women like to be cuddled and told that they are loved after sex. So don't look at porn and think that it is reality. Most men's penises are not that large, most men do not last that long in bed, and most women do not look like that, nor are they turned on that quickly. I don't think the porn is a problem unless you would always rather watch it then have sex with your gf, or it negatively affects other parts of your life. Personally, I generally do not think that reading erotica affects morals. The only problem I have is movies or stories that degrade or disrespect women, because it concerns me when men have no respect for women's bodies.
Erotica is fantasy, and fantasy is healthy. People are sexual beings, and I think it is perfectly healthy. I really don't think that you are betraying your gf. If she knows anything about men, she should know that the large majority of men masturbate to porn and it is very normal. Good luck.

"We are what we think. All that we are arises with our thoughts. With our thoughts we make the world." Buddha

Men will never understand that it lowers a woman's self esteem if she knows you're doing that. She will feel betrayed and left out of your sex life.
She will see it as a selfish act and that she is not included. As if you have friend over for a game of cards and she is not invited to sit down as well.
She will feel jealously over pictures or suggestions of other women. That's just the way most woman are. You need to focus your sexual attention on her and less on what you want if you care about the way she feels.