He might be a bit insecure but I think it's more of a maturity thing. Porn to some guys is very private and to have a GF come out and start jabbering about it can make them get there back up against the wall and start saying silly stuff getting all defensive. Guys, even tho they say they want you to come out and tell them straight, when it comes to personal stuff, they don't like the confrontation. He felt embarrassed that you knew he was looking at porn. He thought stupidly that you didn't know. Being put on the spot, he was unsure whether to trust you, he thought it might have been a trick or something, so he started to give you a bull s hit story. Plus it would be hard for him to believe what you were saying because the last t time you expressed how you didn't approve of him doing it, all of a sudden out of the blue it's ok. But in the same breath you were telling him he better not be involved with the chat, etc.....he got defensive because he felt you were accusing him of doing it.So you can see his standpoint.
Anyways getting over the insecurities, and actually understanding that looking or admiring but not acting upon, is human nature to do so, comes with time, and with time comes maturity. I think you are best to explain yourself better to him why all of a sudden you have taken an interest in talking about it. You have to get him to feel more comfortable, and let him know you are not going to punish him for being honest, but would rather discuss it more so there isn't a need to not tell or be defensive.