I teach 10th graders about sex and I use this approach.
First of all, I begin by elevating the marital act back to its appropriate place. I explain under no uncertain terms that it is nothing short of sacred. I then ask if Marraige is 50/50, 60/40, or some other odd ratio, most say 50/50. Then I explain that it is 100/100. It is the TOTAL self giving of one spouse to the other. Once this baseline is established, I build on it with the marital act. I use the following analogy (borrowed from Scott Hahn)...
Let's say you're in church after mass one sunday praying and a man who is hungry walks in... He then proceeds to walk up to the alter, open the Tabernacle, and begins munching down on the Eucharist. No doubt this would be unnerving...
they I explain how this relates to sexality. Like that man's hunger, man's sexual drive is natural. However, like the Eucharist, The marital act is sacred. Punching bags, bicycles, and weight machines are examples of suitable releases of this tension...
Next, we then discuss the fact that in our society, the marital act is regarded as little more than a marketing tool. As a result, our natural sex drive is overstimulated making it near impossible to control. However, as we know from 1Cor 10:13:
13 Let no temptation take hold on you, but such as is human. And God is faithful, who will not suffer you to be tempted above that which you are able: but will make also with temptation issue, that you may be able to bear it.
At this point I distinguish the difference between what Jesus says is the single greatest act of Love possible and what society would have you believe...
Society would have you believe that "if the love is true, it can't be wrong to have sex" In "fact" sex is the ultimate act of love...
However, Jesus says (in Mt:15:13)
13 Greater love than this no man hath, that a man lay down his life for his friends.
Now we've established marital relations to a proper place to set limits. Typically, we then discuss the difference between affection and sexual stimulation. Any direct sexual stimulation (touching breasts, crotch, buttocks, etc. is direct sexual stimulation and outside of the acceptable bounds of affection. A warm embrace and kissing are acceptable forms of affection. (effectively what you would to to a family member of the opposite sex in affection).
Of courser I did more as a teenager and I know know today's teenagers will do more, but it is important that they are properly armed with the truth so that they can be responsible at least in their thoughts and so that they can have rightly formed consciences.