Maybes there's a patch of the moon dust that they all agreed to never photograph, where they drew all the boobs and dicks.

Imagine: aliens get the Arecibo message but don't quite know what to make of it, until they see the boobs and dicks drawn on the moon and they correlate it with their own crass fascination with reproductive organs, and only then do they recognize humanity as a self-aware species.

That, or they just smudged it out after doing it.