They're works of Art. In those days it was more common than now to be naked. In bathhouses, everyone would see each other naked, and just roam around, and it was perfectly alright. (Although it was considered indecent for a man to be seen in public with an erection).
Besides, back then the upper middle class, or royalty would be the only ones to be able to afford to purchase statues and portraits. They probably have several concubines anyway. It differs from civilization to civilization, but generally, nvde portraits and statues were considered art forms.
When does it stop being art and begin being pormography? By the way it's presented. There still is nvde art today, but it is meant for a person to admire the body, it's contours, skin tone, etc. porm is just plain six. Men with surgically, modified ding dongs, knocking on women with surgically modified jugs. It's not presented as an art form, it's alot more crude, a porm star would have a vulgar pose, such as wide open legs. While nvde models would just be sitting there, or standing there, or whatever.