I believe that sex outside marriage is ridiculous merely on secular grounds. Naturally (being a theist), having God tell us helps immensely, but He is not essential in
this logic. Morals need the Lord, but ethics need not be directed by Divine edict - one tells us what's right, and the other tells us what works with human beings.
My logic behind not having sex before marriage is that abstention of that desire builds moral character. When a man does not engage his will to his most important organ, he learns discipline. When there is not yet marriage to chain and harness a man to one specific woman, discipline is even more important. I believe this is an ethical imperative because the more discipline you have controlling man's sexual spirit, the less capricious he will be in society. When a man is cowed into abstaining from sexual forays before he has only one woman to concentrate on, his brutish nature has been subdued. Imagine if the nature of men was let loose for one whole day, and all men acted on their sexual feelings instead of restraining themselves, as society expects. Why, I have a feeling that many women would regard that day as one of the worst days in their collective lives.
Marriage, as a legal shackles, keeps the man from promiscuity - even if he becomes adulterous, he must be very cautious and minimal in that sin, so as not to alert his wife. When there is not a marriage bond to keep a man on one woman alone, what is stopping him from going to any other women? This process of pre-marital sex can lead in dozens of unsupported babies and many ruined lives from sexual diseases. Anyway, even if a truly loving and responsible man has sex before he is married, the legal bond is not present. It is like atheism and morality, with me; you may choose a good morality, but nothing really keeps you from going to any other morality without God. Likewise, sexuality without marriage, in terms of ethics, may seem stable at one time, but it can really lead to any other woman. Hell is the punishment in the case of the former, and divorce/disgrace/financial ruin is the case in the latter. Analogies between God and anything else are imperfect, of course, but there it is.
Anyway, God said so.