While diverse sexual experience is rapidly becoming the norm for young teens in the 21st century, the best protection for them is the time honored presence of parents who are involved in their kid's lives.
A subject with this many hot-button issues needs a clearly defined context. First, this discussion will consider the entire teenage spectrum, adolescents 13 to 19 years of age. Second, to ensure that readers of different backgrounds are all on the same page, so to speak, this discussion will be constructed around a specific question.
"Is it okay for teens to engage in any sexual behavior?"
The average parent would probably answer that question with an emphatic "No!"
Appropriate Sexual Behavior
Taking a step back, you'll notice that the question asked if any sexual behavior was okay, not sexual intercourse. Sexual behavior is not a specific act, but a spectrum of behavior. An example at the more innocent end of the spectrum could be a couple holding hands and hugging, through wool gloves and winter coats, while walking home from school, on a freezing cold February day.
At the other end of the spectrum is androgen drenched, clothing cast in all directions, gripping the headboard for dear life, intercourse. In between those extremes are many levels of physical intimacy.
In this context, if asked, "Is it okay for teens to engage in any sexual behavior," most parents would utter a grudging “yes”, provided that teens take the time to enjoy the hand holding and hugging for all it's worth and delay taking their gloves and jackets off as long as possible, perhaps until they are in late thirties.
Talking to Teens about Sex
A discussion with teens about their sexuality is going to be just as difficult for them as it is for you. But if you can place the conversation in a context where you look them straight in the eye and honestly say to them that you are okay with them engaging in some sexual behavior, you are going to have their undivided attention. Not to mention that you may come off sounding remarkably hip, without really giving them permission to do anything risqué.
The single biggest misconception about teen sexuality is the idea that talking about sexuality with teens will "put" ideas into a teenagers head. The hormonal restructuring that takes place in the adolescent body during puberty is akin to filling a storage tank with flammable gas. Talk is the valve that allows a safe, controlled release of pressure. How much pressure is released makes the difference between whether teens light up a reasonably safe, controlled, evenly burning flame, or a blowtorch that sets everything within a 50 yard radius on fire.
You may also want to read How to Talk to Teens about Sex.