I do not agree with anyone. This can escalate into some VERY SERIOUS behavior! Approach him (not Mom just yet). This can destroy things unforseen. I cannot believe the other answers are blow offs. Shocked. If smoke is coming from the window of my home; I want to know. If I hear screams from a neighbor; I want to be sure everything is O.K. If I see a child crying, an addict in distress-c'mon-it is our responsibility as human beings to have compassion. No-one wants to paricipate in HELPING others as was once common. Try to help your father. Ignored it may escalate and you will carry a burden. Knowing you, in particular is hurt may prompt him to question his choices. The passive attitude of others is why those who are troubled feel alone.
How do you know your mom doesn't know? It's none of your business. You are a kid. Stay out of it. Your dad is a grown man. Would you like it if he caught you watching it and told your mom? He's is grown. If he wants to watch it then he has every right.
For all you know, your mom knows he watches it, she may too. It is their business. Its not him 'cheating' on her, let her figure it out own their own. Do you really want to be the cause of an argument between your parents? This isn't an uncommon issue in marriages.
Some of the answers given to you will not be helpful. A person who watches pornography could possibly be addicted just like a smoker or drinker. It doesn't necessarily make him a bad person or a bad parent. Ultimately, it is up to the person themselves to seek help for their problem, and they eventually will, IF they recognize it is a problem for them.
I'm so sorry you're having to deal with this. You should probably sit down and talk to your dad about what he's doing. Express honestly how this makes you feel as his child and how your mom will feel when she finds out, because she will figure it out. Some researchers say that the repeated use of pornography can interfere with the ability to enjoy and participate in normal marital intimacy. If left unchecked , what starts as casual viewing of pornography can eventually lead to an escalation to more hardcore, abnormal abnormal material. The course of this problem may be gradual and undetected. Like a cancer, it keeps growing and spreading. It rarely ever reverses itself, and it is also very difficult to treat and heal. Denial on the part of the addict and refusal to confront the problem are typical and predictable, and thus almost always leads to marital disharmony, sometimes divorce. The apostle Peter, himself a married man, urged husbands to assign honor to their wives. A husband who fails to do so will find that his prayers to god are hindered. (1 Peter 3:7) I wish you and your family the best.
I think that you should just leave it be, He has a right to privacy and he is a grown man and can do what he wants. Besides, what are you doing snooping around on his computer? I am sure that if you had a computer you wouldn't want him snooping around on it (which he is completely entitled to) and also, if your dad caught YOU watching porn, I don't think that you would like it if he told YOUR mom.
Forget you saw it and tell nobody. If you confront your dad he could get mad and embarrassed. If you tell you mom she might be angry and confront your dad. If you tell a sibling, they could tell their friends. Don't make a big deal out of it, it's not too uncommon.