What would you do...?

Yes it is a long question and I'll attempt to answer it the best that I can. It sounds like in the beginning of your relationship you had a lot of trust built up for this man who has done nothing for you to think otherwise. So when all of a sudden this lonely coworker decided that she was going to target this "nice guy", his niceness started to respond by saying "well maybe her intentions are innocent", however I agree with you on the fact that he should ignore his curiosity and place his focus more on you. His explanations probably did not make sense because he was "caught" having that curiosity. The problem is when we have curiosity such as this situation, it tends to turn out emotional and we end up making the wrong decision. Welcome to lust. He probably still has insecurities that existed before you even came around since he was cheated on and is working them out by involving pornography into his lifestyle. This has nothing to do with you in which it's not surprising that he does want you involved. For a woman this is devastating because we feel that we are not providing the needs that he seeks and thus we feel undesired. I would start to lay out some consequences at this point. He knows how it makes you feel but he does not seem able to stop which sounds like an addiction. If he denies it until his face turns blue yet his actions are obvious, then yes, it's an addiction. If you offer to involve yourself into his "addiction" then you are ENABLING him when in all reality you are not in favor of this at all thus you are not being honest with yourself. When you have a relationship that involves an addiction, it's the same as him being involved with another woman since she is coming first in your relationship and you feel set aside. So be honest with yourself. Don't place responsibility on yourself and tell him what decisions you've come up with when it comes to how he is affecting the relationship and his role as a dad. What are you going to do to prevent yourself from feeling like you have to be involved?