All of Catholic sexual morality can be boiled down to this statement:
The marital act must be unitive and procreative.
That means that it must occur within the context of marriage, it must be for the union of the spouses, and it must be ordered toward (and thus not thwarted actively from being) the procreation of children. It is to be a life-giving expression of love that occurs within the context of the family. Children deserve a mother and a father in a committed relationship to one another, and so this life-giving expression of love must only take place within the context of that relationship.
Now, before anyone argues that not all couples are fertile, or that not all marriages are happy, I respond that infertile couples or unhappily married ones still act according to the ordering of the act, that is, what its nature is, what kind of act it is. We are not consequentialists. The morality of the act is not determined by its result, but by the object which is acted for, the intention for which it is chosen, and the circumstances of the act. So if an infertile couple, who experience infertility due to a chronic pathology, engage in the marital act as a unitive expression of love within their marriage, and they do not otherwise put anything in the way of the nature of the act being procreative, all else being equal, then they have acted morally.
In short, pre-marital sex is wrong because it changes the nature of the act in some way or another. If it is accompanied by contraceptive behavior, it ceases to be unitive or procreative, in addition to not being marital; if it is not, then it is still ordered toward procreation, and an injustice is committed toward the child who is then not born within the context of the marital union.
This is not to say that unwed parents don’t love their children or anything like that, so before anyone takes offense, realize I’m not making a judgment on the character of any person. I am merely speaking to the nature of an act, to what kind of act that it is.
-Fr ACEGC