Futuresoldier:
For the sake of discussion, I'll concede. The Bible is a pack of lies. We'll start with that premise, and subsequently, toss it out. Fair enough?
You're still left with two absolute and undeniable truths of life:
1) People make choices.
2) Actions have consequences.
Now, why is sex before marriage a bad thing?
The answer is "personal responsbility." You, and you alone, are responsible for the choices you make. You, and you alone, are responsible for the actions you take.
In these days of state-mandated and legally enforceable child-support, a pre-marital affair "could" result in an unwanted pregancy. That "could" mean child-support payments for, oh, at least until the "child" is of age to have completed four years of college. (Yes, this is experience speaking--ex-wife, child-support, etc.)
Then there's the possiblity of stalkers, psycho ex-lovers....
Can you say STD? AIDS?
I'm also certain I'm not the only remaining "old fogey" who still believes in shotgun weddings. My 16 year-old daughter will tell what's going to happen if she gets pregnant before she gets married. She WILL marry the father, period. What happens after that is NOT my concern. But THEY will start a life together, period.
Failure to consider the possible ramifications of one's choices and actions, regardless of how small the chances are of the "unexpected" happening, is a sign of great immaturity. So, you may ridicule these folks all you wish for obeying a "commandment" which could "possibly" be a lie. They are, nonetheless, avoiding choices and actions which "possibly" could lead to a lifetime of regret. And considering that man is, by nature, a creature driven by seeking that which brings the most pleasure and happiness, avoiding the possiblity of pain, when possible to do so, just very well may be the greatest pleasure and happiness of all.
Still, experience is the best teacher. You'll figure it out.