Already in 1916 the  Hungarian psychoanalyst Sándor Ferenczi in Sex in Psychoanalysis wrote:

“The derisive remark was once made against psychoanalysis that the unconscious sees a penis in every convex object and a vagina or anus in every concave one. I find that this sentence well characterizes the facts.”  (tr. Ernest Jones)

I found the above dictum while researching sexual symbolism. Ferenczi’s dictum was most famously referenced in Life Against Death: The Psychoanalytical Meaning of History by Norman O. Brown.

Update 2025: I illustrated this post with The Shell by Odilon Redon.