The answer is that when they married, they freely entered into a covenant of God’s making. They solemnly promised before God and their fellow man that they would exercise caring love for each other from that time until death separates them. They gave themselves, each to the other, totally, without reservation. This is what makes marriage so wonderful. Each person knows his or her own sins and imperfections; each knows that the other has his or her sins and imperfections. Yet they give themselves, each to the other, in caring love, totally and without reservation, for better and for worse, for life. They become “two in one flesh.” This is why the language of the Church generally refers to marital sexual intercourse as “the marriage act.”