Why is this painting shocking?

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This painting, titled Glory Hole, is by Heidi Neff, a painter whose works have been in shows from San Francisco to New York and, yes, my daughter. She has a collection of nineteen large paintings (this one is 60" square) based on church ceilings, where instead of angels and saints, nude figures embrace and copulate in various states of ecstasy or despair. She has written:

My work most often depicts a mass of sexual frenzy, which can be read as an orgy, a last judgment scene, or the descent into Dante’s Inferno. . . . In traditional Christian thought, sex can equal lust­—one of the seven deadly sins. Paradoxically, sex can also equal marriage—one of the seven holy sacraments. This type of paradox is central to the way I think about my work. In my work there coexist attraction and repulsion, loneliness and the quest for solitude, sex that is anonymous and sex that is intimate.

Soil can be life-nourishing earth or just dirt. Spirit can be holy or evil. Anything truly human, however, must be a mixture of soil and spirit. This painting is shocking only to people who want to be all soil, or all spirit.