Well, whatever technology measures or threats are used against teenagers, more often than not it will fail. Humans are curious creatures, that's why we go to explore the space and the unknown, visit different countries, etc. as adults... and as teenagers, it is normal for them to be curious about sexuality. And just like adults who do everything they can to go to space or travel, teenagers will do whatever's necessary to satisfy their curiosity.
Maybe it's just my own beliefs... but I think promotion a healthy understanding of human sexuality is the way to go.
If I have kids, I'd rather my kids develop a healthy and positive view and feel comfortable talking with me about everything, rather than thinking that sex is something to be dirty and to be ashamed of... learning instead from pornography, which isn't exactly the best source of sex education someone can have. I think such views may result in them not learning the facts and potentially getting themselves into trouble, contracting STD/STIs or getting pregnant, when they could have known and done better.
Now back on the technology side of things, if you use Internet Explorer... cookies are stored in two places, and I think deleting one doesn't necessarily delete the other. Cookies will may give you an idea of what sites have been visited. But then again, finding out what site the boy visited does not solve the root of the problem...
[drive]:\Documents and Settings\[username]\Cookies
[drive]:\Documents and Settings\[username]\Local Settings\Temporary Internet Files