That concept has been lost by the libertine-ians.
To which my libertine-ian response goes like this: If governments stopped subsidizing immoral behavior, then immorality would be automatically self-limiting. If drug addiction were punishable by death in some lonely gutter, we would get a lot less of it. Whether homosexuality is "evil" or not will be determined by gays' own will to live, given the public health consequences of their activities. In other words, the threat of HIV causes rational people to modify their behavior; those who don't, die off. It's as simple as that.
That's true, but that's not what I meant.
I meant, any behavior (or "transaction", as the poster so aptly called it) that normally results in pregnancy when males and females are involved, and which routinely results in broken hearts, depression, suicides, knifings and shootings, bar fights, wife/partner beating, drug overdoses and all manner of other antisocial and dysfunctional conduct is not "private".
No one-NO ONE-has EVER understood the sex act to be "private" in the way the poster meant, and for the reasons cited above.