Sounds like you need marriage counselling but your husband is being an asshole.

You have a right to set your own boundaries on what you consider being faithful and unfaithful, but other people have that too. In this particular case, you both need to state them clearly before you can start saying he is intentionally cheating on you. Saying it makes you a little bit upset isn't really clear enough imo. That's not to say he isn't being a shitty partner in this case for multiple reasons, but I just mean it's only cheating in your head. Not his. So until how you're both perceiving the issue has been aligned this thing is going nowhere.

Just a side note... I'm not siding with him in any way or saying you don't have a right to be angry or upset, but your comment about an open relationship was a pretty extreme escalation his reaction is understandable. You can't put watching porn in the same league as exchanging nudes with people and expect them not to be upset when you suggest an open relationship. You fucked up there and if you don't own it and make an effort to deescalate you will be partly to blame for this getting out of control. It doesn't make him any less guilty, but if you still want to fix this, you (now) also have something to make up for.