@Thatonegirl,
I've been in your position and this is my take on it. Many men have a porn addiction and since that is usually more "fantasy" (he will never meet them, can't communicate with them, will have no relationship with them, etc.). I am not saying I agree with it; I am simply saying it is more harmless. The dating sites however, is a TOTALLY different story. Those are real live women he was communicating with, searching out, flirting with (I am sorry, but if you are on a dating site, that is what you are doing). If you caught that, I would also have to worry about sex sites and what he was telling other women (sadly, he may feel guilty and regret it, but there is also a good likelihood he isn't 100% truthful bc he doesn't want to make things worse). Anyway, it is the dating sites and things like that that hurt me MUCH MUCH worse. I broke up with my boyfriend of 2 years after finding out. He called me a month later asking me to go talk to a counselor with him (one who deals with sexual addictions and relationship issues). She was a fruitloop and I walked out after 3rd visit and refused to go back. He found another counselor and we went for both issues and how to rebuild trust. I have full access to his phone, computer, etc. I wont lie- the first year was rough and I had many doubts and no trust at all. We had to work hard to rebuild that trust. 2 years later we are engaged and I trust him. I still have open access to everything and I check on occassion but not nearly as often as I did before. I can actually say that because he so remorseful and knew he screwed up, he worked hard at rebuilding trust and he was sympathetic when I was upset, etc. Our relationship now is probably stronger than it was before. Had he not been willing to be totally open and understand my lack of trust in him, it never would have worked. So I think it depends on HIM and how far he is willing to go to win your trust back. It will not happen overnight and it's a long, hard road but it can be done. He does need to realize that it isn't harmless.... maybe he didn't physically cheat but if he was on other sites talking to other women he probably had an emotional affair and that is very damaging to a relationship. What he needs to realize is if he hid it from you in any way, there was some form of cheating going on. If he expects you to just get over it (bc he doesn't want to deal with it or whatever the reason is), or isn't there for you when you have your bad moments, then you need to get out now. No need to waste your time and prolong the hurt- just walk away. He screwed up so it's up to him primarily rebuild the trust.
If he's in it 100% then also recommend a good counselor (at least for a little while). Like us you may have to try a couple to find the right one but that can really help both of you form a better relationship. Good luck.