I found this on wikipedia well after the song had been written-


The vagina dentata appears in the myths of several cultures. Erich Neumann relays one such myth in which "A fish inhabits the vagina of the Terrible Mother; the hero is the man who overcomes the Terrible Mother, breaks the teeth out of her vagina, and so makes her into a woman."[1]
The myth expresses the threat sexual intercourse poses for men who, although entering triumphantly, always leave diminished.[2]
The vagina dentata has proven a captivating image for many artists and writers, particularly among surrealist or psychoanalytic works. Although the myth is associated with the fear of castration, it is often falsely attributed to Sigmund Freud. Freud never mentions the term in any of his psychoanalytic work and it runs counter to his own ideas about castration. For Freud, the vagina signifies the fear of castration because the young (male) child assumes that women once had a penis that is now absent.[3] The vagina, then, is the result of castration, not the cause of it.