There are a few philosophers who still push the “perverted faculty argument” to prove that contraception, homosexual acts and masturbation are immoral. This argument is based on classic natural law, which is itself based on a metaphysics that assumes essentialism and teleology (broadly Aristotelian).
Because of that, it's usually not taken seriously anymore. It would still be interesting to find out if such arguments even fail at a late stage, when a lot of heavy lifting has already been done (that is, the opponent has granted all the relevant metaphysical foundations).
One version is the 40-page article by Edward Feser: In Defense of the Perverted Faculty Argument (please read at least the twelve pages of part IV. before answering, thank you).
He states his key premise (full argument on page 403f) as:
Where some faculty F is natural to a rational agent A and by nature exists for the sake of some end E (and exists in A precisely so that A might pursue E), then it is metaphysically impossible for it to be good for A to use F in a manner contrary to E.
Since he doesn't want to condemn chewing gum as immoral, he grants that using a faculty F for an end “other than” E is morally neutral.
Hence examples like chewing gum (which is merely other than, rather than contrary to, the natural end of our digestive faculties) […] simply miss the point of the argument.
But chewing gum looks strikingly parallel to masturbation. It even decreases appetite, like masturbation temporarily reduces sexual tension. The digestive system prepares for food, but no food is taken. The reproductive system of a solitary masturbating woman prepares for heterosexual intercourse (or so claims Feser), but in the end, no man has sex with her.
So to finally get to the question:
In the article, is the differentiation between “contrary to” and “other than” really meaningful? If yes, then how exactly should we understand it? If not, can you go into more details how and why the reasoning here became fallacious?
PS: I'm genuinely just trying to understand how people manage to reach such strange conclusions – no intentions to make this an “am I right?” post.