Well, it was originally seen to be very sexually provocative - the plunging neckline of the dress seems to bare a lot of skin, and initially it was painted with the right strap slipped off the shoulder (though there was such an uproar about that from the family and onlookers that Sargent was forced to change it...). Combined with the fact that the woman portrayed, Gautreau, was a well known socialite and instantly recognizable because of her fairly distinctive beauty, despite the attempt at anonymity with the title from Sargent, and you can begin to understand why a relatively prudish French society would be as offended as they were...