Summery of Answer:  

 As a parent I know exactly what you are going through. When you have brought your children up with certain standards it is distressing when you find they have gone another way.   However, we do recognise that we live in a world that has a lot of wrong in it and that there are pressures at school and elsewhere that encourage boys to experiment. The internet just makes it so much easier.

How to deal with it?

There are some Biblical principles to follow.

1. Ephesians. 6:4KJV: "And, ye fathers, provoke not your children to wrath: but bring them up in the nurture and admonition of the Lord."

It is easy to come down hard on kids when we find them making choices that we don't approve of. When boys have reached the age of your sons then they resent a judgemental attitude -- even when you are in the right. I think it is certainly appropriate to express disappointment at their choices but to then to help nurture them into making better choices.

2. The Bible does not talk about pornography. That is hardly surprising as the printing press had not been invented let alone the internet. However it does lay down some principles as to the way to treat people of the opposite sex.

-- One of those principles is that of the submission to one another that we find in Ephesians 5:21-32. In this passage we find a respect for our marriage partner following the example of Christ to his church. It is a commitment of love and understanding. It is the total opposite of the physical lust evoked by treating girls as objects or self gratification.

-- 1 Cor. 6:15-18 talks specifically about the kind of attitudes towards women that will lead men away from Christ.
"Do you not know that your bodies are members of Christ himself? Shall I then take the members of Christ and unite them with a prostitute? Never! Do you not know that he who unites himself with a prostitute is one with her in body? For it is said, "The two will become one flesh." But he who unites himself with the Lord is one with him in spirit.
Flee from sexual immorality. All other sins a man commits are outside his body, but he who sins sexually sins against his own body."

If your son has the Christian values you live by he may want to consi der how his Christian values relate to him looking at images of girls who are being exploited and, in essence, prostituted. He may also reflect on the fact the fact that whatever the mind dwells on eventually can tend to action. Would he really like himself to be featured in one of those MP3 videos? How would he feel if one of the girls on the screen turned out to be his sister, his aunt, his cousin. When we start to realise that the girls in these pictures are somebody's sister, cousin etc. and that they have feelings, are real
people and not just objects, then sometimes it can put things into
perspective.

-- Proverbs 5:3-6 shares ideas appropriate to pornography.

For the lips of an adulteress drip honey,
and her speech is smoother than oil;
 but in the end she is bitter as gall,
sharp as a double-edged sword.
Her feet go down to death;
her steps lead straight to the grave.
She gives no thought to the way of life;
her paths are crooked, but she knows it not.

3) You may want to remind him that pornography does go against the
principle of the ten commandments. Exodus 20:14 states: "You shall not commit adultery." Jesus takes the principle one stage further: Matthew 5:27-28
"You have heard that it was said, 'Do not commit adultery.' But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart."

Your son probably feels guilty about what he has been looking at and this may well make him defensive. It is equally possible that parents over-react and there is some truth in it being "normal teenage stuff" even though parents would wish it was not.

I have seen it effectively dealt with by parents sitting with their son/s and sharing their disappointment, and then asking them how, together, they can solve the problem. Discuss with them the value we place on women, also the addiction that can develop when we keep looking at such images and how that can lead to mentally "undressing" every girl walking down the street. I can't say you will get a joyous reception, but attacking the problem with understanding they may at least listen.

Some parents have come up with an internet usage contract which both parents and children signed.  It may be good to develop the contact with the child rather than impose it on them.  Here is an example.

This may not work in every situation but it can be a success.   To make it work you must regularly check the computers.  If you are not computer literate enough then ask a friend for help you search the computer for unwanted images and files they may think they have deleted.  Let them know that you are watching in a kind and loving manner, and for their own protection.  The ideal is to build trust with them.  Where I have seen this contact in use I have certainly seen a reduction in problems, and in some cases total success.  

Teenagers are stretching their wings. They want independence but at the same time want understanding and respect from their parents. Your love needs to shine through your discipline. Your forgiveness needs to be sincere. They need a chance to regain your trust. I cannot recommend the above as the perfect answer. Each parent needs to work that out within the framework of their own family structure. I can only say that it seems to be a system that has worked for some.

Related questions:  I'm addicted to pornography.  We're Christians but my husband looks at porn.