That all sounds about right. The point I was making is that the poster was looking at it from the standpoint of "reducing harm", whereas in reality they were me

That all sounds about right. The point I was making is that the poster was looking at it from the standpoint of "reducing harm", whereas in reality they were merely following their moral intuitions about what's sacred and taboo in society, and then backwards-rationalizing it through the "harm" moral axis. Very common in the liberal West where we think of ourselves as being above taboos and caring about divinity, except we still do very much. E.g. digging up the corpse of a dead relative and having sex with them doesn't harm anybody, but is highly taboo. Having sex with a dead family pet, doesn't harm anybody, but highly taboo. Siblings on contraception having sex, zero harm, but very taboo. Hentai of protagonists who could be underage, zero harm, very taboo. etc. etc. Women, children, LGBTQ members, the involuntarily unhoused, minorities, Muslims and many others are all currently sacred groups whose sanctity you cannot violate even in a work of art and fiction. So back to my point, it has nothing to do with harm, it's all about sanctity.