Freaky little teenagers and their transition to adulthood, our love for them should not diminish during this time, they are still children and need lots of guidance (even though they already know everything).
Explain how males are very visual, and that by looking at inappropriate content, these 'unholy' images can get stuck in your head. We were not designed to watch others have sex, it is unnatural at the very least, and clearly unholy.
The real question is did you simply take away the computer and say "bad boy, no spanking...", or did you take it further and have a real sex talk. You don't have to get into particulars, just explain the proper context of sex, and sex drives, and the transition to adulthood. A young man is going to be obsessed with girls at some point, the more they know about sex and its consequences the better. I think kids slip into sex without ever having heard an adult's view on it, they get all there information from their friends - and you can just imagine the teen world view on sex these days.
Concerning masturbation, its a dirty little thing that teen boys are likely to do - I think instead of focusing on the topic of masturbation so pointedly, perhaps the global discussion of sex, inappropriate content, and parenthood in the Christian life will be sufficient. And, don't forget to include an explanation of the latest craze: teen girls who target young boys for sex and to become the daddy of their children. There are young girls targeting your sons for just such an agenda; they decide before hand to jump your son, perhaps even with the intent of becoming pregnant, they will lie about taking the pill, so that your son will not use a condom, and then the girl keeps the kid, doesn't marry your son, lives at home with her parents, who do most of the child rearing, so that the young girl can go out and party with her friends - oh, and don't forget that your son will now have his paycheck garnished for the next 20yrs for child support. Obviously they shouldn't be having sex until marriage, but they should at least know the possible consequences.