Thank you all for your input.

I think the 'why' of this thread is simple. Just like we did last Saturday...again we'd be going into the streets again next Saturday. .... And of course we know the problems teenagers usually face (not just 'em alone - including some adults). So, I need to talk about the OP to them. Believe me those teenagers are always ready for questions. One of the questions people usually ask is, what makes it a sin?
A sin steals the good from some action. For something to be a sin, it has to take away something that ought to exist. Murder takes away life, etc.
In the case of sexuality, it's primary purpose is the bringing forth of children; and the second purpose is the aiding of two people in maintaining their relationship. ( Cf: Genesis 1:28, Genesis 2:24-25 ).
Sparrowhawke is correct when he talks about the body learning; A sexual act, and *especially* orgasm, leads to a permanent change in the brain at very least. Such changes can be actively measured using nuclear medicine, or functional MRI; the exact same way that permanent changes to the brain due to certain (not all) drugs can also be measured.
I would recommend "Change your brain, change your life" by Dr. Daniel G. Amen M.D. as a general secular introduction to what can be proven via. scientific method (To be read criticially, of course, which is beyond the thread's scope. )
In any event; sexuality causes two people two become one in the
flesh; and that includes more than just physical juxtaposition, nor does is only mean the child once the two bodies fuse into one. It means, however, that the bodies of two people have become inextricably entangled.
The factual findings of science, over time, converge toward the conclusions found in scripture regarding what is damaging to us; and what is good for us -- and give a new depth of understanding to those who are stable enough to study both science and scripture carefully. If you plan on being a resource for youth, it might be a good idea to study the secular side as well -- for inevitably youth tempted toward atheism are mixed in to such groups.
Within scripture, we're told: Man was made from the slime of the earth:
Genes 2:6 But there went up a mist from the earth, and
watered the whole face of the ground.
Genes 2:7 And the LORD God formed man of the
dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul.
The image here is that of the fine dirt particles wetted (slime) being formed into man; and of that same material a woman is also made.( as a modern science didactic... Man's slime has X and Y chromosomes, but when God took from the body of the man, the woman only got the X part. )
Even today: When a man and a woman come together, we know they share (gasp) slime.
Given these general thoughts; I think you might be able to see why the ancients understood the slick molding material of the potter (clay/slip) and seminal fluids to be of the same purpose; for later in scripture we are told "God is the potter" and "we are the clay". We are also told that God forms the child 'in the womb" (and what is wetter than that?!) etc.
The important thing to grasp, is that the substance of a man and woman transfer during the sexual act -- and the two become united. It can not all be "washed" away afterward in the shower. The memories remain and the changes to the brain remain; I find that men most easily understand it when told that each person one has sexual relationship with -- you really do leave "a little bit of yourself" with each of them; and your mind will return to them.
The questions of contraception will undoubtedly be raised as a "solution" to such problems; but it fails.
The mind is still changed, the person will still dream of these others even if they forcibly put them out of their mind during the day. Moses was so aware of this, that although he did not brand wet dreams "sin", he none the less made men wash and stay out of the camp for a period of time when they had them: Leviticus 15:16, Deuteronomy 23:10.
The very purpose of sexuality is to make the bodies of two people "cleave" together permanently (Genesis 2:24, Matthew 19:3-10, etc.); So that any attempt to have sex without full completion -- merely triggers the mechanisms in the body meant to bring people together, but without a proper mate to *stay* with forever.
False triggering of the mind is the general problem found in all kinds of lusts; for, one might examine a less controversial lust: Lust of the stomach (gluttony). And the same issue arises, people will think -- I can just empty my stomach.... but any study of the issue will arrive at another problem caused by that purging (Bolemia and anorexia); not to mention that we know there are people starving in the world for which wasting food shows contempt for.
I think of it this way:
Jesus said that some demons can only be cast out by prayer and fasting; His saying applies very much to sexual temptation -- prayer and abstinence is *necessary* to remove demons from power over the body *prior* to marriage, or else they will have the opportunity to interfere with the marriage.
A man and a woman can come together in a natural marriage; but if one wants to be joined by God (permanence) then more is required. God will not join two people who break his commands.