"I know it when I see it." That was Supreme Court Justice Potter Stewart's attempt to explain how to define hardcore pornography in the 1964 case Jacobellis v. Ohio. Is what you're seeing here pornography? Flat, geometric shapes, in various shades of pink and white.
But is that a black triangle, or a patch of pubes? A pink circle, or a nipple? Apple's app reviewers see the latter — or at least see us seeing the latter, our mind's eye reconstructing these pieces into naked bodies and bodily fluids. So they rejected the app, thinking that "many audiences would find [the] app concept objectionable."
But what's dirty here — the app or our brains?