What is sin?

Heinlein was probably being glib. I 'd guess that the question of “necessity” has to do with self defense.

The Gide quote has a broader range of implications. I can picture some sort of New Age Nuns reprimanding children for “obscuring the soul.”

I stopped reading Heinlein after “Stranger in a Strange Land,” which left me with a peculiar feeling.  I liked some of his more adventurous, youth oriented novels I was all of twelve or thirteen years old and the semi magical, innocent character of Michael intrigued me.
Much later I realized that he was also a foil for Jubal Harshaw's “sophisticated” morality or (lack there of).

I read Andre Gide's “The Immoralist,” during the latter part of high school. A friend who went to a different school started recommending it and saying it was intense and claiming that the book influenced him.  He wouldn't say just how, but reading the book, I was a little shocked. Basically the protagonist recovers from grave illness by embracing a new  philosophy of hedonism and perversity…He cultivates an interest in Arab boys, not only as erotic stimulus, he also condones and even cultivates a certain amount of criminality

Of course, I knew that Gide wasn't advocating immorality. In part, he must have been seeking to justify his own sexuality.
There is a point when Michel, Gide's protagonist, makes quite a bit of sense, embracing life and putting aside the narrow morality of his society.  The point at which he goes to far was not clear to me, but in the end his wife is afflicted with the same  tuberculosis which he had recovered from, and Michel seems incapable of helping her or really caring for her.

My friend was always “going through something,” while reading Dostoevsky or some tony book.  He did have an appetite for exotic drugs and a kind of twisted relationship with a close female friend of mine. After I went to college, he seduced my sister, and later started getting her involved in fairly large drug deals. There were also some Dotcoms that appeared to have no other purpose but attracting investors.
At one point he appeared to be inviting me to join him in group sex with my former girlfriend. My fiancé was pregnant and sleeping in the next room at the time, but I had other reasons for declining.
I have to conclude that he was pursuing a path of his own choosing similar to Michel's.

Today, I do appreciate Gide much more, but back then “The Immoralist” seemed to justify the same 'open' sexuality that led my ninth grade English teacher to molest me. A doctor did the same thing to me around the same year that I read that book.

My friend left the country shortly after nine eleven. He complained that America was without intellectuals. He stayed in London for a while and was last heard of in Spain where he was part of a Techno-Trance collective.

Both “The Immoralist”, and “Stranger in a Strange Land” are dangerous books in that they suggest abandoning most of what we call morality. Heinlein panders a little to a libertarian male audience, and I think he has a clear idea of what would make the world a better place.
Gide is more honest in exploring the character of a man without morality. He does not preach, or even judge Michel in any way but leaves it up to each reader draw their own conclusions.

Another version of the liberal creed:
“”Every thing is cool, just don't stand on anyone's head”
I heard this attributed to Jerry Garcia, but i can't remember  where, He may have said this from the stage at Woodstock.