If he is using it as a substitute for having sex with you, it's a problem.
If he starts asking you to do stuff that he saw in a porn video and it's not your cup of tea, it's a problem. (If the new stuff is a turn on, then it's all good).
If he starts treating you differently or doing things that are out of character between the sheets, and this new stuff turns you off, it's a problem. (If it turns you on, then it's all good).
If you have asked him to stop and he promised he will, but keeps watching ... then you have a problem.
The best ways to stop it is #1 don't let him know that it Get under your skin. # 2 let him catch you watching men when he say something to you about it make a deal when he stops you will also. # 3 watch it with him if he has a problem with that then you know its more than watching the porn.
You may not like my answer to this, but it is the ONLY answer. Your man needs more sex from you. And since he's looking at porn rather than actually cheating, he's a visual. Which means you could dress up a little bit for him once in a while, get some lingere if you can and suprise him once in a while. I know we dont always have time for all that, but if you don't put your relationship first, who will? The internet.. or possibly another woman? That shouldn't be an option. You basically need to give him what he's looking for so he doesn't have the need to look for it. hope that helped.