You're talking about the Bobo Doll Experiment? It shows that children imitate what they see, which is why some things need to either be explained properly to them, or kept away until they are able to understand them.

Something to keep in mind is that kids will imitate everything they see. They'll want to wear daddy's suit or mommy's makeup, they'll play fireman, truck driver or chef, and they'll imitate sex too if they see a sex scene. Imitation is not exclusive to violence.

Another thing is that children have a need to understand violence, especially because they understand it is harmful and thus scary. Every kid will hear about criminals in real life who hurt people, and the fear that a stranger will hurt them or their family will be with them.

Play-violence as well as fictional violence on TV helps them explore and understand violence in a safe manner. We fear what we do not know, and our biggest fears often diminish when we better understand the subject we fear.

As for sex, the problem is that it is not presented in a healthy way on TV. Nudity is there to arouse (otherwise they would hide it - no, I absolutely do not buy the "nudity for artistic reasons" excuse, and at least graphic violence has the honesty of saying it is there to give the audience a thrill), and sex is often portrayed as a casual activity.

You can have parents explain to the kids what sex is, but the problem is that unlike with violence, kids will not have healthy ways to explore sex.

For example, a 14 year old can take martial arts where an adult will teach him about fighting in an appropriate way. However, a sexually curious 14 year old would not be able to be introduced to sex by an experienced adult. That would leave the 14 year old with only the options to experiment on his/her own, or to fool around with equally inexperienced and ignorant friends of the same age - both of these things can be unhealthy and damaging, and lead to an incorrect understanding of sexuality.

For this reason, kids and teenagers should not be exposed to depictions of sex that are arousing or that portray it as something to desire (it should be limited strictly to scientific/medical discussions, like you would find in a sex ed class).

Basically what I'm trying to say is that kids and teenagers shouldn't be tempted with sex because they don't have healthy ways to indulge their curiosity. It's good to educate and inform them about it, but not to tempt them and make them want to try it. It's different with violence because kids have options to explore that safely.