What does 'Wouldn't throw a rock in a school yard' mean?

Going through old photo albums with my 99 year old Gramma she pointed out one of my grandpa's uncle's & said this.  I was not sure why anyone would throw a rock in a schoolyard but I asked why this uncle in particular- it's a pretty obscure saying, but it's a very polite & modest, though playful, way to say someone is promiscuous. She explained that they "wouldn't throw a rock in a schoolyard" for fear of hitting one of their own illegitimately fathered kids, as any of them could be theirs.

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