The mutual self-giving of one man and one woman makes sense only within the bonds of matrimony wherein each one gives the gift of self to the other, mutually and exclusively, until death do them part. This mutual self-giving is so powerful that it can bring about a new life into the world. In any other setting, it would be a lie. 

One cannot say that self-giving is total if the two parties involved are not bound to each other through marriage, or if one or the other party is married to someone else. How does one give oneself totally and exclusively to each other when there is no matrimonial bond involved? The act would be a lie, too if the married couple has recourse to contraception. How would the conjugal act be a total gift if the couple refuses to be open to the possibility of welcoming a child?