[dominik]

The key lies in the phrase "mutual self-giving."

Catholics see sex as a mutual gift of self between spouses. Through the language of their bodies, the husband tells the wife "I am yours," the wife tells the husband "I am yours."

As a gift, it is necessarily permanent. It's more than impolite to give a gift and then ask for it back. It's also exclusive -- you don't give someone a gift and then take it back and give it someone else -- the original recipient would rightly be aggrieved at such a turn of events ("Hey, you said that was mine!")

While all of marriage subsumes this gift of self, sex forms the act -- the sign -- signifying the gift. Sex is a physical expression, in the concrete language of the body, of the mutual self-gift that the husband and wife made to each other in marriage. The human reproductive organism consists of two persons: a man and a woman. Their union thus not only signifies their mutual gift of self to each other, but also serves as a creative act, a procreative act that generates life. Thus sex is both unitive and procreative, or in vernacular alliterative, both babies and bonding.

All that leads the Church to say that "the deliberate use of the sexual faculty, for whatever reason, outside of marriage is essentially contrary to its purpose."

[jimbokun]

That is an eloquent expression of a beautiful idea, and I'm not even Catholic.