The general program of the (Vatican II) Catechism is to explain how the blessedness of human life is a sharing the blessedness of the divine life and, to this end, they explain five of the commandments as finding their source in divine attributes. The fifth commandment arises from God’s dominion over the earth (as Plato argued in Phaedo, we are the possessions of the gods and so cannot kill either ourselves or others) the seventh commandment arises from the benevolence of God who has disposed goods in an order that must be respected, and the eighth commandment arises from the fact that God is truth. The account of the fourth commandment makes “The divine fatherhood is the source of human fatherhood; [which] is the foundation of the honor owed to parents.”
The commandment against adultery, however, is unique in being based on the interpersonal communion of the persons of the Trinity. This marks the only time where a commandment is based on God as known, not by reason, but by revelation. Here are some hypotheses to consider in light of this.
1.) Teachings on sexuality are more dependent on revelation than other teachings. Given their foundation in God as Triune, sexual teaching is harder to get to the bottom of by reason alone. This does not mean attempts to base sexual morality on natural law will be unconvincing and ineffective, but they will always come with the sense of leaving out something fundamental.
2.) Sexual morality manifests the Trinity more profoundly than other aspects of morality. Here the beatitude “blessed of the pure of heart, for they shall see God” takes on a much deeper and more literal meaning. On the basic creedal level, the first procession of the Trinity comes forth by birth and the second comes forth by the union of persons as a single principle; and the two great mysteries of the faith are mysteries of birth, i.e. the Second Person is born of the father and then conceived by the Holy Spirit and born of the Virgin. The nexus point of this mystery and the world is through the Eucharist which is, again, the act of a father.
3.) There is a sexual dimension to all vocations in the Church. Here it is important to point out that the second thing the Catechism says about sexuality is that it is ordered to procreation but “in a general way to all human relationships and to all vocations. How else to explain the Church placing a crucial importance to sexual differentiation in religious vocations (e.g. its insistence that all monks are male)?
To develop this point is the most constructive response to the “contraceptive mentality”. The Church does in fact allow for a sense in which sexuality can be developed outside of the demands of procreation. The religious life does this. It is not asexual (in which case the absence of female priests could have no basis) but transcendently sexual. Sexuality is ultimately ordered not to orgasms but to the physico-spiritual unity of persons, and this happens in a more perfect way in the religious life than in the married one. The confection of the Eucharist is an obvious way in which this is the case.