This is extends a comment I made here.
In ethics, Thomism or Catholic natural law theory is a system of reasoning whereby moral conclusions congruent with whatever is convenient for Catholic doctrine at the time are deduced from the metaphysical structure of the universe.
The Thomist view on sex is that the inherent function of sex—what brought it into existence—is reproduction, so sex can only be used according to the form of reproduction. The originating function of sex determines what is the proper, natural, way to engage in sex. Men and women were brought into the world for each other and only in their complementarity can moral erotic relationships exist.
This reasoning thus reaches the conclusion desired from the metaphysical characteristics of the universe: sex and gender must conform to the simple binary identification of remaining as born and only having sex with the opposite sex within marriage based on us being fully defined by our genitals. This excludes all the queers (gays, lesbians, bisexuals, transgenders, intersex) who do not conform to this simple binary identification. But it (at least, it does in the modern, amended, version) allows married couples past conception age to continue to have sex (since it is the reproductive form that counts, as that is congruent with the defining function of sex, not whether conception is possible).
The mechanics of sex thus become a major moral issue because sex can only properly happen by the form which conforms to its defining, which is to say its originating, the reason why it exists, purpose.
Any sexual activity that fails to conform to that reproductive form is using sex against its originating purpose. So, no blowjobs, no anal sex, no masturbation, no contraception that interferes with the natural process, no sex within the same gender: the only permitted intentional orgasms come from a penis ejaculating into a vagina connected to an unimpeded uterus. That sex in nature has lots of other functions does not matter: they are not evidence, they are malfunctions and malformations.
Fairly clearly, this is using the conclusion to select the premises. One simply focuses on that part of reality that conforms to the desired conclusion and dismisses the rest as improper: hence the ability to “deduce” from the metaphysical characteristics of the universe the desired conclusions.
To see how opportunistic this is, consider the case of the pig leg. The inherent function of a pig leg is to move the pig around, which would seem to ban bacon, ham and roast pork, given they involve irretrievably frustrating the pig leg from carrying on the function which brought it into existence. When we make and consume bacon, the pig leg is not being used according to the form that arises from its originating function.
Clearly the pig leg can perform other functions than moving the pig around but, then, so equally can sex perform other functions than reproduction. Certainly, we have to eat to live, but we are not compelled to specifically consume pig legs. It is just tasty and convenient that we do so.
While we can use other things than pig legs to get tasty nutrition, sex provides catharsis and intimacy in ways that generally lack equivalent substitutes. Whatever difference one is going draw between making bacon and non-reproductive sex, it is not going to turn on originating purpose.
The real difference between the originating function of sex and the originating function of pig legs is that Catholic theorists are looking for ways to restrict sex (for reasons I discuss in my recent post on the role of God in moral discourse), but they are not looking for ways to restrict food. (If originating purpose determines proper use, we should only eat things which can continue to function when we harvest the eaten bit, or which can no longer perform their original purpose. It is also hard to see how evolution can be a proper process, since it is all about finding new functions.)
The religious taboos are driving the reasoning, not the metaphysical structure of the universe. Which, as noted, is the trick with such "natural law" moral reasoning and what makes it so doctrinally useful: it picks on the aspects of reality it finds convenient and simply dismisses any divergence as "improper". (That this then ends up creating “improper” humans in significant numbers each generation is just too bad; they grow up as isolated individuals in a sea of "proper" humans, so have historically been very easy targets.) If the conclusion gets to choose its premises in that way, it is hardly surprising that it ends up "deducing" from the metaphysical structure of reality whatever conclusions are currently congenial for Catholic doctrine.
But it is also a specific example of a more general issue. When we theorise, we abstract away from reality. In doing so, we can abstract away from the reality of people’s humanity. Which is precisely what Catholic natural law doctrine does, it abstracts away from the reality of human sexual diversity thereby, as I noted previously, creating categories of people who should not exist—the queers: all the people who do not conform to the binary identification of sex with gender.
Since they clearly do exist, generation after generation, the response is to deem them metaphysically flawed, for the theory says they ought not to exist, and require them to conform to the theory, at whatever the personal cost (and blame them when they do not so conform). Their inconvenient existence is then a burden placed on them: the theory is trumps. This is, of course, an approach we see in other ideological systems. It is oppressive—even murderous and tyrannical—there too.
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