The Christian idea of marriage is based on Christ’s words that a man and wife are to be regarded as a single organism—for that is what the words ‘one flesh’ would be in modern English. And the Christians believe that when He said this He was not expressing a sentiment but stating a fact—just as one is stating a fact when one says that a lock and its key are one mechanism, or that a violin and a bow are one musical instrument. The inventor of the human machine was telling us that its two halves, the male and the female, were made to be combined together in pairs, not simply on the sexual level, but totally combined. The monstrosity of sexual intercourse outside marriage is that those who indulge in it are trying to isolate one kind of union (the sexual) from all the other kinds of union which were intended to go along with it and make up the total union. The Christian attitude does not mean that there is anything wrong about sexual pleasure, any more than about the pleasure of eating. It means that you must not isolate that pleasure and try to get it by itself, any more than you ought to try to get the pleasures of taste without swallowing and digesting, by chewing things and spitting them out again.
Mere Christianity, C.S. Lewis
This has got to be one of the best and most concise explanations for why sex before marriage is forbidden for Christians. But it still may be hard for people (even Christians) to understand because our society no longer views mankind as an integral (inseparable) union of body and soul.
Many people today act as if the soul does not exist at all, but then would contradict this by saying they believe there is a soul. The reason for the inconsistency seems to be the the false philosophy that is promoted by various forms of media and politicians: the whole separation of church and state mentality. The effect of this on our understanding of the human person is that we compartmentalize what we believe in the public sector and what we believe in the private sector, which is where religion has been relegated to. This false notion is then reinforced by so-called scientists who are really just material reductionists that deny anything spiritual because they cannot see it, touch it, or smell it.
Christians, on the other hand, are called to see the world and mankind as it really is. We were created by God as a hylomorphic union of body (matter) and soul (form): the one cannot exist without the other. (* see note) From this correct understanding of mankind we can see that what we do with the body does in fact have an effect the soul, because you cannot separate the two. And when you use your body in an improper way, as in sex before marriage and especially homosexual activity, then the negative consequences can be felt in your entire being.
Now some people may protest at this point and claim that they feel no negative effects from premarital sex or homosexual activity. I suspect that some of these people, if they were honest, would admit to a certain hollowness within themselves. For the others that do not admit to this, we can explain their lack of remorse on a society that for decades has been filling our minds with the mantra “Oh come on, it’s ok. Everyone is doing it.”
But Christians know that is not true. It is not ok. When the Church says that sex should be saved for marriage (just to clarify – one man and one woman) she is not trying to take away anyone’s fun. She is simply trying to help her children to be truly happy, which can only happen if they are living their lives in accordance with the Truth. And the truth in question here is that mankind is a union of body AND soul. Understanding this and helping others to understand is of great importance.
*Note: At least in this world they cannot exist separately. On the other hand, the Saints in heaven, the souls in Purgatory, and those in Hell exist in a spiritual state until Christ returns and everyone receives their bodies back. God created our souls as immortal and so they cannot cease to exist, but our bodies can and do die as a result of sin. This existence of the soul without the body should be viewed as a Divine concession that has been allowed until all things are restored when Christ returns.