Here is my current theory on the Sheela-na-gig. I believe they were markers for places of prostitution or where prostitutes could legally (church sanctioned) stand and sell their services.
Now a more insidious theory would be that the Sheela-na-gig was used to trick or lure pagans into churches because they did not know the symbolism or how to read.
My second theory is that they are a horrible creature that can suck people into themselves or other dimensions using their vaginas or a vagina-like opening.
Not all sheela-na-gigs appear to be female. On one hand, this could reflect the fear and fascination with hermaphrodites. The symbols could have served as a warning against entering a certain area or perhaps as assurance of safety in a particular place.