It's real.
But it's not.
You two are in a big grey area.
She's right that it is like when you look at pictures of women.
You are right that it is not (exactly) like that.
She is right that "that's what you're here for" (a sex rush while online/viewing pics).
You're right that she could meet up with this guy.
All the other people are right that these thing do tend to move on to real life sex. (kinda like the ole wts said about holding hands; technically they were right)
She talks to, and fantasizes about, another person doing sexual things to her.
You look at, and fantasize about, another person doing sexual things to you.
I think you two have a tuff road ahead, but maybe it is just the road that, realistically, every couple in this modern age is going to have to face. You are going to have to figure out exactly what level of sexual "loyalty*" (for lack of a better word) you are going to agree to in your relationship.
When you turned to porn, that is a step away from most** peoples view of absolute sexual "loyalty" to their marriage mate. Yet many people consider that the norm for married men. You may argue that it is morally ok, or that she knew that about it all along. What you can not argue is that your looking at porn is absolute sexual loyalty to her.
It may be that you two will disagree about how to solve this. She may just want you to get comfortable with it. You have to solve it, one way or the other. Obviously you can't go through life keyed up about this all the time. Well, I suppose you could, but that is not life, that is hell.
She may have been fine to just decide on absolute sexual loyalty before she tried this. She may feel differently now.
As Janh said, "Any woman thinking a man -- any man -- goes through life thinking of sex with her only is seriously deluding herself". And vice versa. So "absolute sexual loyalty" is a pipe dream anyway. Which means one has to at least negotiate from the knowledge that sexual thoughts about other people will at least be part of your partners sexuality.
Me thinks your problems go way deeper than sex. I'd say get some help in the form of a dispasionate 3rd party family counselor. You guys are definitely going to need a referee for this game.
*fidelity seemed to easy to split hairs about
**guess I can't really speak for most. maybe most witnesses?