I am sorry you have been dealing with this, its really unfair for him to get himself into situations where there is a high probability that it could turn into something more.
With that being said, you have to talk to him.........and soon. Before he has a chance to make the biggest mistake of his life. Waiting to see if he does it is not a good idea, many men often do not see what they are risking until after they have taken that leap so my suggestion is to step up before that happens.
I would suggest that you set up a time where you can talk to him in a calm rational mood. Wait until after he is relaxed from work, and not stressed out about anything. Then instead of asking him (Because if you ask, he is encouraged to lie) be straight forward and tell him what you know. Let him know that you know about the porn, the my space and the women. Be firm, but also remain calm and try not to get angry. Showing him your disappointed and even hurt is fine.....but try not to get so emotional that you cry. When you become this emotional you are likely going to be lose your temper and may become confrontational which will lead to him shutting down.
Hopefully he will be straight forward and honest about it......again do not pose your questions to him in a way that will allow him to lie. Let him know what you know, then ask him why or what his intentions are. Let him have time to speak, this is important. If you dominate the conversation it will seem more like a lecture and less like you communicating with him.
And finally let him know how all this made you feel and where you want to go from here. Can you trust him to change? Or is he likely going to simply change his pass words and start again. If so then you may want to lay down some ground rules....such as he must go to marriage counseling with you and he ends all contact with this women.
Now after that is not the time to sit back and relax, let him know this is serious and could be a deal breaker......while you should not use it against him he should know that this is VERY serious and not something you intend to take lightly. He should know that you are upset by this and that you expect real change out of him.
I know this is hard, but sitting back and allowing it to go on isn't going to help matters at all. You must deal with this like any other bump in your relationship and demand commitment from him.