The backside of Art

I learn via Literary Review that someone has published an entire book centred on Renaissance art which heavily featured the male backside:   Seen from Behind: Perspectives on the Male Body and Renaissance Art

The review could be better, but this part, talking about one painting which does sound pretty ridiculously butt focussed - The Massacre of the Innocents by van Haarlem - is pretty amusing.   Here's the description:

Take The Massacre of the Innocents by Cornelis van Haarlem, which dominates one of the magnificently refurbished galleries of the Rijksmuseum. Painted in 1590, it is a scene of tumultuous violence, anchored formally by the massive nude figures of four soldiers in the foreground, one striding towards us from the right with a dead baby under his arm, one flat on his back on the left, overcome by a group of mothers, who gouge out his eyes. Counterpoised in the centre are two soldiers seen from behind, one standing, the other, biggest of all, down on one knee as he cuts a child’s throat, his colossal backside not only in the viewer’s face but also inches from the face of the child’s desperate mother. The heroic scale of the picture, some eight feet by twelve, adds to the interpretative puzzle for a modern eye: why make a vast male arse the focal point of a major religious painting? It’s impossible, too, not to wonder if the Dutch, whose art embraces the everyday, the suggestive and the downright lewd, kept a straight face about it, then and afterwards.

And here is the painting, which does, indeed, seem to comprise some very oddball composition:

Why, exactly, the rampaging soldiers are nude, and flesh coloured, while the mothers appear alien grey, must be another puzzling question for art historians.

I wonder how impolite it is for adults to giggle at this when viewing it in the art gallery?  Pity the school teacher taking groups of kids to see it, too.