This is a term that Leo Steinberg crafts to describe a fixation he identifies in Renaissance art on the sexuality of Christ. Related structurally to the more common Latin phrase ostentatio vulnerum (literally "the showing of the wounds"), which refers to the Biblical episode in which Christ appears to St. Thomas after his resurrection and brandishes the wounds he incurred during the passion, ostentatio genitalium designates a visual convention in which Christ is depicted showing his genitals. Steinberg's choice of the word ostentatio, or "show," is important, as it indicates a conscious choice to display which animates the pictures he discusses. The fact of Christ's sex, so obvious in the works under discussion, cannot be attributed simply to increasing pictorial naturalism among Renaissance artists, for it is not merely depicted as incidentally present, but as being presented. The Madonna, Steinberg points out, frequently "unveils the Child or decks its loins with attention-gathering ceremony" (pg. 35). This theatrical showing, this literal ostentation, is of as great a theological significance as the canonical ostentatio vulerum, according to Steinberg, as it served during the Renaissance as one of the primary pictorial vehicles for expressing the miracle of the incarnation, the divine occupation of human flesh.
Cosimo Rosselli, Madonna and Child with Angels