The French court was notorious for adultery, lesbianism and sodomy. A surprisingly permissive attitude towards all these had been established at the French court as far back as Marguerite of Angouleme, sister of King Francis I, in the 16th century. She said that “unhappy is the lady who does not preserve the treasure which does her so much honor when well kept, and so much dishonor when she continues to keep it.”
The treasure was her chaste virtue, incidentally, not – like – a pile of kryptonite. A pile of kryptonite you should always keep. Rabelais was so charmed that, despite being a shameless misogynist, he dedicated his ribald epic Gargantua to Marguerite.
During the court of Francis I in the 16th century women began opening their bodices down below their nipples (unless they were small busted, in which case padding was inserted beneath their stays, just as if they were at Victoria’s Secret) to catch male attention at court. And they complain about female chauvinist pigs buying into male expectations now.
There was nothing surprising, then, about the fact that the Princesse de Lamballe was showing off nipples in her picture. It was actually pretty tasteful that she was only showing one.