" I told him if I ever catch him on those sites again that is it. "
In other words, you told him that he's got away with it this time, and he needs to hide it better from now on.
You've nailed it ^^
He'll just hide it better. You said if you catch him and all he has to do is make sure you don't catch him and it's very easy to do this.
Experienced serial cheaters can have:
A secret credit card (online only statements)
A secret bank account (online only)
A secret phone
A secret email account
A wash bag at work
I've no issue with porn in the least, but registering on the site 3 weeks after you got married would likely be a dealbreaker for me.
I don't want to think that you really want to know the truth, because it's frightening.
If he had used an escort, would it be the end? Would you really want to know. If so tell him you're unsure about his story and you'd like him to take a polygraph. His reaction will probably be a giveaway.
Of course one's marriage resorting to polygraphs isn't great, but I personally would rather the whole truth now, than finding out 3 kids later, when I could have divorced and moved on to a better relationship.
You don't actually need to go through with it, but as long as he thinks you will, he's either going to confess or try and beat the test.
Having read the other side so often, of married men who have confessed online to cheating throughout marriage and before, there must be lots of betrayed wives who have no idea.