This is in reference to da Vinci’s “Virgin of the Rocks.”
http://news.yahoo.com/s/ap/20100715/ap_on_en_ot/eu_britain_da_vinci
Back in the day, there was some controversy over this painting, due to the phallic rocks, even being anatomically correct and all. Da Vinci was thus compelled to paint a non-heretical version in order to pacify the church, called “Madonna of the Rocks.” The tame one is in London, and the heretical one is at the Louvre in Paris and appears in “The Da Vinci Code” by Dan Brown. Some might see the primary reason for the sexuality surrounding the virgin as a contrast to virginity itself, thereby emphasizing it and bringing the viewer to think more deeply about it. But obviously not the church as an oppressive institution. See: lisashea.com/hobbies/art/madonnarocks.html