Hey here is a sketching challenge.
I dare you to sketch your genitals. Take a pen or pencil and first sketch a male or female genitals from your imagination. You should generally know what it looks like. Now once your are finished your quick sketch, take a look at it from a distance and ask your self if this sketch matches your own genitals.
the point of this is, we humans see our own genitals everyday, unless we are obese. We also see our feet everyday too. But for some reason it is a lot more easier to sketch our genitals then it is our own feet or face.
Now if you really want to practice sketching, sketch your own genitals be looking a a mirror.