I don't have kids, so I can't really say what we do at my house (googling "sexy girls" is not only allowed on my home computer, but encouraged

)
Here at work, I am pretty much in control of the filter that we use, and we try to take a non-draconian view of the filtering, but if it was my kids, I would definitely filter, as well as not allow unsupervised access to the computer while the internet is on.
Boys will be boys, and I can't even imagine how maddening it is to try to raise one of us, but the internet can be a crazy place, and I don't think I would want to turn a kid loose on it unsupervised.
I would think that one easy way to solve the problem of him knowing how wrong that stuff is, would be to call the oldest one in, and ask him to show you exactly what he was looking at, and make him retrace his steps in front of you. For me, that would be one of the most embarrasing things that could happen, much worse than when my mom found my dirty magazines. It won't cause him to stop wanting to look, but it will make him think about why it is wrong to objectify women that way.
I think the best thing that you can do is make certain that the internet is off when you aren't there. If you aren't sure how to accomplish that, you need to learn, because you can bet that they will. Keep the router in a locked closet, and when you leave the house, the router gets turned off. Also, don't allow them to have computers in their rooms, make them be in a community room that you can walk in or be in at all times, and make certain you check in on them constantly.