![](http://2.bp.blogspot.com/_o93AaY0GzH4/TDLvIhPXJKI/AAAAAAAABqE/nFZsizFFqCQ/s400/Le+d%C3%A9jeuner+sur+l%27herbe+_+French,+The+Lunch+on+the+Grass,+1865,+by+Edouard+Manet.jpg)
The naked woman was the reason. After the post below it is funny to think it was a scandal but at that time naked women in paintings were not a contemporaneous woman as this one. She is a woman of that time and this is one of the reasons people thought it was immoral.
If she was a goddess there was not problem but she stares at you without any shame and how dare her being with two men?