Speaking as an unfortunate product of the American education system, where some schools do resort to abstinence only sex education, let me be as blunt as I can in saying that those who advocate only for abstinence are not just ignorant, but are doing a LOT more damage than good.
While it is true that people shouldn't be running around having sex with every single thing that moves, there are some good reasons both to have sex before marriage, and to avoid the techniques that are used by abstinence only advocates.
1) Speaking from a psychologically theoretical standpoint, the romantic relationship can be broken down into three parts which all contribute to the relationship (as described in the Triangular Theory of Love). These are Intimacy (the feeling of closeness and friendship), Commitment (that one's obvious), and Passion (which ranges from physical attraction to physical action). The strongest relationships are built by combining the three elements, while the weakest rest only on one. That said, sexual compatability is an important part of a relationship. To paraphrase Lewis Black, anyone who decides to wait until marriage to have sex is a gambler on a level I can't even begin to comprehend. Sex is an important part of the expression of love in a relationship, if you wait until you're married and then find that you and your partner aren't compatible in bed, it can be worked around, but it's just not a plesant situation.
2) The fact is that people are going to have sex, and this is one area in which abstinence only advocates to a lot of damage. By refusing to teach children methods that prevent pregnancy and the spread of STD's, they fail to teach the children who inevitably will have sex how to protect themselves. If you want to see this in action, realize that the rate of transmission of sexually transmitted diseases is rediculously higher in the US (where many schools do have abstinence only programs) than it is in European countries that have comprehensive sex ed programs. Kids here don't even realize that there are contraceptives that they can use if they have sex, which is why pregnancy rates and STD transmission rates are higher in the US (and even higher in Red states where abstinence only education is more common than in Blue states).
3) Abstinence only advocates frequently use religion to scare children away from sex, and this is one of the many areas in which religion causes a massive amount of psychological damage. Many children coming out of this system, are so disgusted with their own sexuality that they have to spend years in therapy to overcome the problems this sort of indoctrination causes, and some will even physically punish themselves, cutting or worse. Considering that sexual desires are inevitable, as we all have them, and that religious systems frequently threaten that those who give in to sexual desires will literally be tormented in fire for the rest of eternity, it is no small wonder that the pattern is as harmful as it is. The really sick thing is that adults do this to innocent children who blindly accept whatever they are told.