If it was private, how is it that they can be prosecuted in the first place? Every criminal sodomy case that I've seen referenced on this board since I've been here has not resulted from a private act. Since you can't be referring to anything going on in the present, were you referring to something that someone else suggested?
There have been a few cases in which people have been prosecuted for sodomy despite efforts to keep it private. In one case, for example, (IIRC) police entered an apartment on a no-knock raid on a drug tip; I don't remember if they found any drugs, but they did find two men engaged in sodomy whom they arrested for that.
IMHO, any anti-sodomy statutes should be written so as to only apply if the participants had reason to believe they might be observed by unwilling spectators, and should apply as well to indecent heterosexual activity.