It's not just TV. It's magazines, books, the internet, toys, clothing, and as they grow older peer pressure. I'm fortunate that my lo is only two and she's quite happy to draw pictures or go out for a walk but I know that the older she becomes the more difficult it will be to shield her from these influences.
There is explicit content on TV on virtually every channel at all times of the day (classic examples of this are programmes which are usually shown after the watershed being repeated in the morning) and I think the worst channels of all are the music channels. Some of the videoes on there are just one step short of pornography! Teen magazines are a problem too. When I was a teeneger (in the olden days) magazines had things in like how to get rid of spots and kissing a boy for the first time. Now it's 'position of the week' and how to look like a 20 year old. Childrens books are not the innocent Famous Five and Secret Seven stories they once were and I just hope that my daughter doesn't get into 'Bratz' dolls. The internet can also be a problem because no matter what you do or how many parental controls you have in place there's always something that slips through the net and let's face it kids are far more clever with computers these days and can probably override most control passwords.
I think one of the biggest things of all though is peer pressure. Girl's in particular can risk being ousted out by their friends if they're not wearing the right clothes, shoes or make up or if they don't dye their hair or look like a supermodel. We live in a very image conscious world where people can't/won't accept people for who they really are. I'm sure there are plenty of girls who have little interest in fashion and make up but go along with it so they don't get left out by their friends. The same principal applies to underage smoking and drinking. And then there's the 'Who's done what with who' where the most shocking thing will gain you the most popularity, or put another way if you haven't done anything yet then you'd better start or your friends will think you're boring.
Another thing i'd forgotten about is film age classifications. A film that say 20 years ago would have been rated as a 15 or an 18 will now be 12A so just about anyone can see it as long as they're with an adult.
As I said before it's easy to shield younger children from these influences but as they get older and more independant it's an absolute minefield and you can't oversee what they are doing/watching/reading all the time.