Going off the Rattata and original Bulbasaur pictures, I believe the idea was to have Female and Gynomorph-(boobs and penis)/herm pics. This may have simply been for the ease of quickly adding "male" images though?
While Bulbasaur originally had a female pic for females, the ivysaur and venusaur pics did include the penis regardless of sex, and an update made that the case for bulbasaurs too.