First let me say this your teenagers are going to engage in sexual activity regardless of whether you tell them not to or you don't. Sex and sexuality are their decision not yours.
As an intelligent caring parent you need to talk openly about sex, the physical, mental and emotional consequences of it and how to protect themselves from unwanted pregnancy, VD's, and emotional trauma of being used.

What's that?

I think you need to be more specific. What ages are you talking about, and sex with whom? If you're talking about the kind of stuff most of us did as kids- the 'show me yours, I'll show you mine' stuff then that's all about natural exploration between kids. However, if you're talking about adults introducing kids to sexuality, then of course, no way.