I'm not really sure.
It doesn't really literally apply to actual teeth more a fear to do with male vulnerability.
I think it tends to be more representative of the wet, scary, dark place with teeth than an actual vagina.
Inviting, mysterious and dangerous.
It's a fear of sudden castration/emasculation/pain that probably has something to do with guilt derived from pleasure and the idea of "what is the very worst thing that can happen?"
I don't believe that any guy has ever wondered truly whether a vagina would bite his dick off or not but Freud is talking about our unconscious fears. And our unconscious fears are not rational.