It has been said that much of the sexual ethic of the Catholic Church can be found in the natural law. That is, it can be arrived at by reason alone, without reliance on God revealing things that we could not know on our own accord. If you want to give reasons to a son or daughter as to the why of the sexual ethic you are demanding of them, it is often good to be able to fall back on natural law reasoning apart from revelation. While revelation is actually a more powerful reason, it demands a level of faith that a child may not have reached. Finally, revelation and natural law cannot be contradictory. If they seem to be, one or the other is being incorrectly interpreted.
Assertion: Human beings are the most important things in the universe. If you have to wreck your car to avoid hitting one, you are considered evil if you do not. If the choice is between shooting a human or a bear, the right choice is always the bear. Mountains, trees, baseball, movies: all must bow to the needs of humans. Other humans are worth going through great inconvenience for.
Therefore, the act that creates humans must be treated with dignity and respect. It is not something akin to racquetball or dancing, something done merely for fun's sake. It has nobility that is not lessened by its ubiquity.
Secondly, new humans must not be called into existence without the proper support network ready to receive them: that is, a mother and a father who are committed to rearing them for life. It is even a worse crime to call new humans into existence and later kill them because they were inconvenient. Abortion is a monstrous crime because it places convenience over the lives of others.
Assertion: Sex promotes bonding. It cements people together more strongly than mere enjoyment of the event would indicate. Not only does it create an emotional bond, it seems to anesthetize one to the failings of the other. It is not that you do not perceive the other's failings; they just don't seem to bother you as much as they otherwise might.
Therefore, having sex is not something you want to be doing when trying to evaluate potential mates. You need to be level-headed and perceptive, not anesthetized to a potential mate's failings. How many bad matches happen where you ask, "Why did he marry her?" and the answer is, "It was the sex"?
But just as you want to be clear-headed before marriage, a little analgesic action after marriage is good. Marriage is difficult. Being somewhat blinded to the faults of your mate is good if lifetime marriage is the goal.
Assertion: Sex loses some of its bonding ability when that bond is made and then broken many times. This is supported by the research that says having sex with multiple partners before marriage is indicative of a higher divorce rate. Conversely, those who have sex only with their eventual marriage partner have a much lower divorce rate.
Therefore, sex before marriage is dangerous in as much as it will weaken sex's binding properties within marriage. How about if you "know" he is the one? He is the one when he stands in front of his family and friends and pledges himself to you. Before that, you are playing roulette with your life and his.
Assertion: Sexuality has a lot of strings. These strings can be tied to the oddest things. In 19th-century China, foot binding was the rage. Women of culture had their feet bound as girls so their feet could not grow normally. The toes were curled under and the feet left small and misshapen. Misshapen, smelly feet were erotic for Chinese men of the age. Were these men crazy? No, they were men just like the ones walking around today. Your sexuality can be tied to an enormous number of things. These strings are there to be tied to your mate.
Therefore, leave your sexuality untied until you can tie it to your mate. Premarital sex, pornography and masturbation all tie your sexuality to things that are not your mate. This keeps the sexual bond from being as strong as it could be. Remember, marriage is difficult. Wouldn't it be great if the things that turn you on are the look of your wife, her breast size, her legs, her type of underwear? You want the quirks of your husband, his way of speaking, his odor, his caress, to be what set you off. It is much more difficult to think of leaving someone when their being screams sexual allure at you.
When your fetish is something besides your spouse, you will tend to want your spouse to conform to your fetish, usually with poor results. People want to be loved for who they are, not for pretending to be someone or something else. You are then stuck trying to rewire yourself, which is always much harder than wiring yourself correctly to begin with.
Assertion: Sex feels great and can be extremely habit-forming. Sex is one of the few things in life that can actually live up to its billing.
Therefore, sexual habits are some of the hardest to break. Forming the habit before you can licitly indulge in it means you will be driven again and again to hurt others and yourself. It becomes much harder to say no after habitual masturbation has started. It is much harder to stop after having sex with a partner than to say no the first time. Indulging a little bit usually leads to indulging a lot more than originally expected.
Assertion: No one wants to be treated as a means to an end, an object to be used. Everyone wants to have his humanity appreciated. People are the ultimate ends, not objects. One of the things that throws sand into the gears of a marriage is when one is treated as an object: "I am just a maid to him." The desire not to be an object is nowhere more visceral than in regard to sex.
Therefore, good sex must be kept at a higher plane than pleasure, which is self-centered. It must have a nobler goal. The creation of new life and the strengthening of the bonds of marriage are called for here. These goals ennoble the sexual act, taking it from the use of the other for your own enjoyment to the procreation of new life and the purposeful bonding of yourself to the other. The fact that it also feels good is a side benefit, not the goal.
N.B.: Agreements where people want to co-use each other as objects almost never last. As time goes on, people change and one always feels cheated. This can drive sex from being a binding thing to a divisive thing. This is one of the pernicious aspects of the contraceptive culture: It can split spouses with the act that should be their greatest glue. This is borne out in research. Spouses who do not use contraception have a minuscule divorce rate.
Assertion: Homosexuality is unnatural. This is so obvious that it is odd that it needs to be defended. When you look at the plumbing, Tab A goes into Slot B. Homosexual relations cannot generate children even between two people of normal fertility. Simply because some people end up wired that way does not mean it is natural. One who is wired that way is wrongly wired — and this wrong wiring is a torment, so much so that we call it a cross.
Acting out homosexually will only drive one deeper into a same-sex fetish (see above). People who suffer same-sex attraction can and do get married. Tying as many of their sexuality strings to their spouse as they can and seeing the sexual act as a noble endeavor within marriage can soften the cross they bear. A spouse and family are a great good to a person and society. Acting out the homosexual lifestyle will close off this great good to a person.
Summary: The facts — that humans are supremely important, that sex binds, that sex is addictive, and that sex can be tied to things mean that sex is a fire that must be kept within the fire ring called marriage. The prohibition on "treating people as objects" means that even in marriage, sex must be for higher purposes: procreation and the marital bond. Homosexuality is an unnatural condition whose effects can be ameliorated by the proper approach to sex.
Each one of these points can be expanded and investigated at length. In a Catholic perspective, they should be followed up with what the Church teaches and the evidence in Scripture and Tradition.
