I am going to give you my opinion on this although it differs radically from that of the majority. First off, it sounds like, from this post and a previous one that you have on the forum, that he is not merely perusing porn when the mood hits him. It sounds like he has a full-blown addiction to pornography. Whether a woman is OK with her man looking at porn or not, it is a problem when he invests more time in his porn habit than in the real, live relationship he is in. Judging from the time that you have suggested he spends with it, this guy probably needs some help.
I don't believe that men are animals who have no control over their actions. We all have urges that may or may not be socially desirable, but we have the choice of whether to indulge ourselves in them or not. If his porn habits make you feel bad, and he knows this and yet continues to behave the way he does, without putting any effort whatsoever into curbing his habit, he might as well be holding a sign that says: "I don't care about your feelings. My wants and desires are what matter in this relationship." Yes, men like to look at porn. Yes, it's normal. I like to eat ice cream. I mean, I REALLY like it. It's a normal thing for me to do. Does that mean that it's OK for me to sit around eating ice cream all day when I know that my partner is going to be repulsed by the gluttonous behavior and by the physical appearance that the behavior will create? Sure, I CAN do it. I'm a grown-up, and it's my right. But I love my partner and I care about his feelings, so I don't do that.
Some women are OK with their men looking at porn. Some women even enjoy viewing it with their men. That's fine. It is not a problem or issue in their relationship. Your guy's porn viewing is making you feel that you are less than attractive and unimportant to him. Your feelings are not right or wrong, they just are what they are. Either he needs to make changes in his behavior or he is not the guy for you. Consider it a blessing that this issue is coming to a head (pun not intended) before you guys have married.
I do not like for my husband to look at porn. I have some pretty deep trust and self-esteem issues caused by some events in my life that far preceed his presence in it. My issues are not his fault. However, because he loves me, he embraces my strengths and my weaknesses, and I do the same with his. He's a guy. He likes porn. His looking at porn causes me to feel unattractive, unvaluable, unloved, and betrayed. Right or wrong, this is how I feel. Because he loves me and does not want me to feel that way, he puts my interests above his own and chooses to fight his urge to look at porn. It is a sacrifice he makes on my behalf. This is not to say that he never has the urge or that he never gives in. It just means that he makes a concerted effort to not engage in an activity that damages my feelings.
If your guy will not change his behavior out of respect for your feelings, and you cannot tolerate the continuation of his behavior, then even though you have invested many years in this relationship, it may be best for you to let go of him. I personally would not stay with a man who held so little value for my feelings and my contributions to the relationship. However, in the end, only you can decide what you can put up with.