Nope, and while welshman's website probably says all this and more, let me just spell out what I would believe are the most important reasons:
1. It brings the relationship into a level of total intimacy before the couple has actually fully made the commitment on a soulish and spiritual level. And since the couple has not yet been able to make the actual step into marriage, it means something is wrong there somewhere, or at least they haven't dug deep enough into the relationship yet to find out if something is truly wrong and they should not be together. Once they have embarked on a sexual relationship, they are going to be intimate whether they are truly ready or meant to be together or not, which will lead to great pain and heartbreak down the road when they find out they were not actually meant to be.
2. It destroys one's testimony. For non-Christians, this is no big deal. The world views it as acceptable for people to engage in non-married sexual relationships, so no problem. HOWEVER, they hold Christians to a much higher standard, often spitefully, and even if they don't voice their disapproval, many will view the Christian who is engaging in premarital sex as undisciplined or unprincipled at best, and a complete hypocrite at worst. This makes it much tougher for that Christian to then turn around and be a "witness" for Jesus Christ, when it's apparent that they are not walking in obedience to the Biblical command to marry in the sight of man.
I recently recall seeing someone post something somewhere, maybe here, about this question. He was a young Christian man in seminary (Bible college), and he was already engaged in a sexual relationship with a young woman who also went there. They both seemed fine with it, supposedly as I recall because they planned on getting married "eventually," but he wanted to know why he didn't feel more convicted about it if it was such a big deal to God.
I never replied, but my answer would have been that he actually was being convicted about it or he wouldn't have been asking the question. The problem is that while such a couple might have made a commitment in their hearts to one another before God, they have not made such a commitment to one another in the sight of man, which is what the Spirit is and will continue to convict them about. It is supposed to be a public declaration, i.e. that he is hers and she is his now, and that all concerned and everyone in society at large needs to respect that fact. Without this public declaration, those attracted to the man will say, "Well, I don't see a ring on his finger yet," while men will likewise see her as fair game if she is showing any interest.
And marriage is so emotionally involving that it is no laughing matter. Countless are the murders that have taken place over jealousies when hearts were broken, so it pays to go into any sexual relationship with a full understanding of the potential gravity of the matter. And if a couple can't get to the altar so that the sex is legally binding, they could be setting themselves up for real trouble way too prematurely.