Though back in those days they knew what the symbol actually meant, it was chosen by its simplicity. The ichthys is actually a symbol of fertility. In today's culture it is displayed on its side to look like a fish. Originally it was displayed with the football-shaped hole pointing upward and the open end downward. This showed what it truly represented, the vagina and anus of the fertility goddess Atargatis. Atargatis is also known in different cultures as Tirgata, Aphrodite, Pelagia or Delphine.
"I see he's pro-vaginas."