This is absolutely WRONG!
First, medical science has shown that a person that has regular orgasms can add years to their lives. Apparently a healthy expression of one’s sexuality has a health-boosting effect.
Second, it has been shown in psychological and medical studies that masturbation leads to more focus upon sexuality, to more frequent masturbation, and to becoming physically sexually mature faster than our mental and emotional faculties are prepared to handle. The same is true for underage fornication; it causes the bodies of young people to grow up too fast, and they are not emotionally or mentally ready to handle it.
Third, sexual intercourse is meant to happen within the bounds of marriage, and marriage is meant to provide a safe place for a sexual relationship and for the raising of children that might result. (not to mention the fact that in marriage spouses are to encourage each other in holiness…)
Fourth, medical science has also shown that sex does not require all that much energy. It makes evolutionary and biological sense; an act which may produce offspring (i.e. children) should not so tax the organism so as to render it vulnerable. The amount of calories burnt up in an average encounter of sexual intercourse is actually quite low.
I haven’t actually hunted up links to back up my rebuttal, but these are just a few of the interesting scientific tidbits I came across (and theological premises too) in my years of reading about studies pertaining to the human body. I just couldn’t not post my rebuttal to Black Angel, when so much of what they said is so out of touch with reality, scientific and theological. If anybody is actually interested, I will try to hunt up some links for this.