d00d -
Why is it such a no-no for kids like myself (i'm 16) to see sex, yet so acceptable for us to see violent movies?
I think that one practical reason might be that if you see sexual activity, you're going to want a piece of the action, and there could be negative consequences, either emotional ("she belongs to ME!") or physical ("Here's your kid - get a job!") that can have a devastating effect on your life.
If you watch violence, it is likely that any inclination you feel to go get a piece of that action will be offset by your sense of self-preservation, so you WON'T do it.
Stated another way,
In the case of sex, your normal drives will push you toward it and there will be few if any alarms going off in your head telling you to stop, and (one would hope) the object of your attention will also be encouraging you to "get it on." You both could get caught up in something that could lead to your harm.
In the case of violence, the object of your attention will be trying to preserve his own hide, and when you get your nose splattered across your face, your own sense of self-preservation will tell you to STOP.
ALWAYS take personal responsibility for preventing unwanted pregnancies!