BRUSSELS — For four centuries, the opulent, exuberant nudes of Peter Paul Rubens have been known to shock and delight in sometimes equal measure. And now, even in 2018, his Baroque paintings can jolt the internet. Belgian museums are uniting in protest against Facebook since they cannot promote Flemish Masters including Rubens at will for fear of falling foul of the social media site’s adult content rules and automatic censorship. “The bare breasts and buttocks painted by our artist are considered by you to be inappropriate.