When sex is done for its own sake as an act of the passions, it is vain and emptied of its dignity, separated from God. It becomes chaotic with all possible acts treated as permissible. It destroys self-control and thus all morality. The act designed to join you to Divine Love, then separates you from God's Love, like Holy Communion (the intimacy between Christ and His Bride) received in a state of sin, that which was meant for a blessing becomes a curse.
When sex is done as an act of Divine Love and the body's participation in the exchange of Divine Love, then it is dignified and serves its proper purpose. It becomes ordered and submitted to God's purposes for the flesh and mankind. It creates self-control, and with it, theosis. Limiting sex to its purpose (as an exchange of Divine Love) dignifies it and makes it serve as the means for the spouses to be joined to God through the Sacrament of Matrimony, making it the part of the sacrament where the three (God, husband, and wife) are the most joined together. A physical exchange symbolizing and being a part of the exchange of Divine Love that already exists in the spouses and was already strengthened by Rite of Crowning.