What if ancient people just enjoyed drawing and sculpting penises and boobs like we do today?
That isn't the case. Where we take an icon to represent fertility there are reasons for doing so (a known fertility god is being represented for example).
But in many cases people just drew things for the same reasons we do now. Amusement or belittling somebody. Take a look at some pervy graffiti at Pompeii of a woman riding a man;
Just one of many many examples of this sort of thing.