I see where you’re going with this argument, and it makes sense on the face of it.
There are cases when expelling through the mouth (vomiting) occurs, but this is only an ordered part of the body’s functioning when there is some form of sickness, that the body is trying to protect against.
Of course, there are other examples – if one is sea sick, or simply disgusted by what one is witnessing, one can vomit involuntarily.
So you would probably argue that it is disordered to cause oneself to vomit, as an act of will (unless you know you’ve ingested some toxin). This wasn’t what the digestive system was ordered to do.
Presumably, even spitting out one’s food would be “not what the digestive system was ordered to do”, although I’ve heard cases of individuals who would like to enjoy the taste of a “naughty desert” and want to have the best of both words – chewing up a chocolate cookie, then spitting it out. Others, “bulimics”, would chew it up, swallow it, and then purge themselves of it.
Food is also a good analogy because the body “eats to live, not lives to eat.” We find pleasure in food as a way of compelling us to ingest it, you could say (just as sexual arousal exists to compel us to reproduce).
There is so much “stretching of the boundaries”, though, when it comes to the human relationship to food. That is, there tends to be what one could call a “morbid” focus on maximizing taste, whereby the taste of the food becomes something of an end in itself.
Two examples of really pushing the envelope, regarding ordered eating, would be the ingestion of alcohol (let’s say vodka, not wine, because wine could have a “health justification”) and the ingestion of sugar-laden deserts.
Smoking cigarettes would be an even more extreme example, because a cigarette not only has no nutritional value, but one is compromising one’s respiratory system. Your respiratory system was ordered to breathe in air, not smoke.
Chewing gum is another example, which I believe was brought up already. Some people like to chew hay, just to have something to do. Some children like to suck their thumbs, even though their thumbs don’t belong in their mouth
Speaking of the sense of touch – which brings us closer to sexuality – a massage is a largely gratuitous act. One can, indeed, get a massage simply because it “feels good.”
A kiss is neither ordered towards reproduction, nor towards eating (despite its involving the lips). It is a display of affection, not necessarily ordered towards reproduction (though it can be).
There seems to be a gap that I still think cannot be bridged through reason alone, and that is the gap between what is “outsmarting nature”, and what is immoral. I just don’t know why it should be compelling, to say that non-reproductive sex is morally wrong. It could be compared, in its way, with non-nutrious eating – the ingestion of empty calories, for example.
One could always make health arguments – as one certainly can, vis-a-vis smoking not only being disordered vis-a-vis the respiratory system, but harmful to one’s health. I still don’t see “unhealthy” to be necessarily synonymous with “immoral”, however.
Even there, though, it cannot always be maintained that same sex contact is harmful to one’s health. A kiss certainly isn’t; nor a caress; nor, not to mince words, anything that is done with one’s mouth or tongue.
Anal sex has a harmful component, though the extent of its harm is in the eye of the beholder. Chili peppers are pretty hard on the mouth, and can make one’s stool burn. One is punishing one’s body, in a way, when one is eating extra spicy. You are getting your body used to it only in the sense that you desensitize yourself to it.
Anal sex, I’m sure, can have serious health consequences, if it’s…let’s say…practiced in a certain manner. Then again, alcohol – if ingested in a certain way – could lead to liver failure, or to death through alcohol poisoning. The alcohol itself has no nutritional value I’m aware of, and most individual’s bodies rebels against that first drink, and even that first cup of coffee, just as it does against that first cigarette. The body says, as it were, “I don’t want this; I don’t need this.” And yet many of us partake in it, nonetheless, just for the pleasure of it. The pleasure of it becomes an end in itself, detaching food’s pleasurable function from its nutritive function.
Again, all of this can be potentially unhealthy; I just don’t see how, in and of itself, it is necessarily immoral.