Women are the keystone?

I stumbled across this in my research today.

"Sheela Na Gig, also spelled Sheela-na-gig, short form Sheela, a type of (usually) stone architectural figure of uncertain significance, representing a naked woman gesturing to or otherwise flagrantly displaying exaggerated genitalia."

https://www.britannica.com/art/Sheela-Na-Gig

https://www.knowth.com/pdf/sheela-na-gigs.pdf

Here's an excerpt from a book listing many of them:

https://books.google.com/books?id=Xjp-AgAAQBAJ&pg=PA155&lpg=PA155&dq=sheela+na+gig+keystone&source=bl&ots=875dkcwZ2Z&sig=ACfU3U1-4NjDIsIOcw4Xub3-mD-MONna0Q&hl=en&sa=X&ved=2ahUKEwj6webNo8PrAhURVc0KHfLjBwY4FBDoATAFegQIARAB#v=onepage&q=sheela%20na%20gig%20keystone&f=false