Men look at p0rn. It satisfies curiosity and yes it can become addictive. Women often misunderstand and think that a man looking at p0rn means that he's somehow unsatisfied or that he's using it as a substitute for the woman he's with. I've been in perfectly happy relationships and still looked at p0rn, it wasn't because I felt my relationship was lacking. It was simple curiosity and in my case it wasn't a compulsion or an addiction.
That said, I have met women who's husbands were indeed addicted and collected p0rn in great volumes. I do not know if those men used it as a substitute for their partners. I have also met men who start out looking at fairly tame p0rn and then slowly escalate to ever more hardcore images. Obviously this isn't a good ting either. I cannot help but wonder if that escalation causes them to lose interest in their mate and pedestrian sex.
P0rn can be used in a healthy manner. It can give couples a spark of desire, it can also give ideas for bedroom activities. Sometimes it's even amusing!
What is it about this activity, in this case, that is a problem? Is it that he's being secretive? Is it that he's being evasive? Do you feel inadequate or replaced somehow? You've tried to restrict him to only watching it with you - why? Is he not allowed to masturbate either? Does he impose such restrictions on you?
It sounds like an open an honest conversation needs to take place. I'd start by dropping the confrontational aspect and by not labeling his explanations as "excuses". Talk to him honestly, don't corner him with accusation. Find out if there's something in the relationship that's not satisfying him. Watch some of what interests him with him, ask questions, try to get involved a little. Try to remove some of the stigma and perhaps he won't feel the need to be so secretive, maybe he will be more honest. Find out if there's some unfulfilled fantasy going on perhaps, discuss it openly without being judgmental.
Relationships are partnerships. Being confrontational and combative isn't going to make things better. If you're hurt by what's going on say so calmly, discuss it, find out what's going on without cornering him. If you corner him he will become defensive, he will lash out, he will retreat. You're partners and you don't want to do that to him anymore than you want him hiding things from you....
I hope that provides some insight. FWIW I don't find p0rn to be "evil" or unhealthy normally however I'd imagine those involved in the industry are probably a little messed up - not a job I'd want anymore than being a stripper! Given a choice I prefer something with a plot or completely amateur but different strokes for different folks obviously...