[quote="VeritasLuxMea, post:3, topic:289268"]
OK. Why doesn't it hold?
I think you're over simplifying the natural law arguments somewhat in the analogy you're trying to draw. (I assume you're trying to draw an analogy to the natural law arguments that homosexual activity, masturbation, or something similar is wrong.)
It may be helpful to examine the sin of gluttony. From Wikipedia:
St. Alphonsus Liguori wrote the following when explaining gluttony:
"Pope Innocent XI Odescalchi has condemned the proposition which asserts that it is not a sin to eat or to drink from the sole motive of satisfying the palate. However, it is not a fault to feel pleasure in eating: for it is, generally speaking, impossible to eat without experiencing the delight which food naturally produces. But it is a defect to eat, like beasts, through the sole motive of sensual gratification, and without any reasonable object. Hence, the most delicious meats may be eaten without sin, if the motive be good and worthy of a rational creature; and, in taking the coarsest food through attachment to pleasure, there may be a fault."
So grabbing a candy bar when you're hungry (or even when you're not particularly hungry, but when whatever food value there is will be put to use) may be fine, depending on motivation etc - but eating to experience the flavor and then inducing vomiting is not fine, because in inducing vomiting you circumvent the purpose of eating entirely.
Likewise, engaging in sexual activities is only a problem when one does so for the pleasure but avoids the purpose entirely. (Notice I haven't mentioned any body parts yet, it's more than "the mouth is for eating" etc.) Sex between a man and a women who are already married is fine, because it is ordered towards creating children. Contraceptive sex is not fine because that purpose is consciously circumvented. Homosexual sex is not fine because that purpose is being ignored entirely. Again, no body parts mentioned.
I suppose you could argue that some of those acts aren't really sex as such, and so aren't covered by that reasoning. But this would be along the lines of arguing that "eating" a cheeseburger by chewing each mouthful and then spitting it out isn't really eating. and so isn't prohibited by the prohibition of gluttony. Gluttony is pursuing pleasure of taste without regards to the purpose of eating, likewise sexual sin is pursuing sexual pleasure without regards to the purpose of sex.
And again, this argument does not need to reference body parts at all.
So: the argument against kissing you provide fails because it's simply not complete (it happens not to be completable, but it fails because it is not complete). You have not demonstrated that the mouth is not to be used for any purpose that completely ignores eating in the same way that eating is not to be used for any purpose that completely ignores nourishment or that sex is not to be used for any purpose that completely ignores procreation.