My guess....

In the late Eighteenth Century that the decent, learned, and God-fearing men who wrote the damn thing didn't think it was necessary because it was generally understood that one either just didn't indulge in what they would have considered gross depravity or would have the common decency to keep it behind closed doors and unspoken about. That would be my guess...YMMV You'll find nothing in any of "The Federalist Papers" either. Certain things back then just weren't openly discussed in society - polite or otherwise. Would any of the Framers specifically want it protected? Some probably would, some definitely wouldn't have. In fact to this day its hard do get a legal definition of what obscenity really is because there's no objective standard. Mr. Justice Potter Stewart observed that very issue when he opined "I know it when I see it." He's a killer and a gunman and he'll make you bleed, shoot'cha in the back or in your sleep.... got no name or identity,..... livin' like the wind he's free!