Also worth commenting is that everyone (including I think Emily Austin too) struggles with finding the right word for "not natural and not necessary." I think we all -- including Emily -- agree that the real issue is whether the activity produces more pleasure than pain. That's a question that is so fact specific to circumstances that it becomes very difficult to determine what "natural" and "necessary" really means except by setting out a specific set of facts.
And for the sake of appreciating that complexity we should all take a second to revel in the appropriateness of Don's hostility to hypotheticals!
How do you set out a hypothetical "natural" and "necessary" that is actually useful without referring to the facts of a particular situation?
"Necessary" to whom and for what? "Natural" for whom and for what? I think there are common sense answers that most of us would agree on, but common sense also probably is what warns us about trying to be too legalistic with what natural and necessary means.