The media in America do not appreciate arguments from faith. In order to engage secular culture, it is often necessary to use reason apart from the revealed truths of the faith. That's okay. On all sorts of moral issues, philosophy provides solid answers without reference to religion.
Specifically, let's take a look at natural law. Natural law theory states that there is an ethical law built into nature, just like there is a physical law in nature. That natural law means that things operate according to their natures (which is a very ancient, well-established principle of secular philosophy). Consequently, the nature of anything has an implicit law on how that thing should act. A rock just kind of sits there because that's what rocks do. Their nature is solid matter, more dense than air, but inorganic (or at least not alive). Consequently, they just sit there. A plant grows and flowers because that's what plants do.
Among the many things that humans do according to their nature is sex. The entire gay "marriage" debate comes down to sex. Sex is what marriage exists for (I'm not talking merely about the sexual act as the purpose for marriage - there is more to sex than just that - but the sexual act is the most direct demonstration, so I will use that). The sexual act does two things:
- Unites two persons into one couple, by which union such persons each transcend their "I" to become "we."
- From that unity, sex creates more persons.
Marriage exists because it is a social necessity built into human nature. From the beginning, throughout the world, we humans realized that the unitive and procreative powers of sex were incredibly wonderful or terribly destructive. Marriage exists as a natural social structure in which a sexual relationship - the sexual act, romance, and all that we find in such relationships - can take place. The children that form the family stem from the union of two spouses. Open marriages, divorce, adultery, and other attacks on the unity of husband and wife harm the dignity of each spouse, who have entrusted themselves to one another, and the dignity of their children, who derive their being and identity from that union. So sex is central to marriage. This is why it is called the consummation of marriage, because marriage serves sex and sex is a way of living out marriage in its fullest meaning.
In order to see the nature of sex, we have to ask ourselves what it does. Fortunately, we already answered this. Sex (1) unites two persons (2) in the procreative act. No one can deny this. Sex obviously unites two persons. They are physically joined. What they are joined in doing is the procreative act. They are joining their reproductive organs, the natural purpose of which is procreation.
So here comes the big conclusion: to use the reproductive organs for something other than their natural purpose would be an abuse of those organs. You can have unity, but if you do not have the procreative act, then what you are really doing is not sex. Since you are not engaging in sex, you are engaging in an abuse of the natural functions of the reproductive organs and therefore a violation of natural law.
Likewise, if you have procreation without union, you are not really having sex, and are misusing the sexual faculty against the nature of sex.
Examples of common acts which abuse sex via a lack of the procreative aspect: contraception, homosexual activity, masturbation.
Examples of common acts which abuse sex via a lack of the unitive aspect: open marriage, divorce, adultery, bigamy, masturbation, rape.
Some acts lack both aspects.
- Scientific studies show that many other animal species regularly engage in homosexual activities. Therefore, it is natural. This is a very common objection, but it falls flat on its face. First, humans have a human nature, and so the natures of other species are really irrelevant. Second, all this shows is that other species have members with homosexual attraction. No one is debating whether some humans have homosexual attractions. We are debating whether those attractions can be followed morally. This argument assumes that humans should always follow their instincts and desires, which is evidently a very destructive idea.
- Scientific studies show that some species can switch their sex between male and female, so gender is really not as rigid as you suggest. Whenever this does happen, notice that it is always so that procreation may occur. The frog-genome-injected raptors in Jurassic Park didn't switch from male to female in order to have homosexual activities with the other raptors. They switched in order to serve procreation. This argument actually shows how uniform nature is in orienting sex toward procreation. It is only humans who try to keep sex from being procreative. As for the rigidness of gender, this doesn't apply to our scenario. It is not possible in human nature to switch from one sex to the other.
- Studies show that children are just as well-developed in homes with same-sex parents as they are in traditional homes. Be careful about "studies" cited in any argument such as this. What criteria did the researchers use? Did they base development on a child's self-esteem? Tolerance? Physical health? We all have different values for which we would test if given the chance. Many of these studies are very subjective and can easily be tilted to one side of the debate or another.