Perhaps because they had available the testimony of Mary herself?? Remember, she was entrusted to and lived with St. John for many years until her death.
"What evidence could St. Augustine possibly have had four centuries later that Marys womb was 'inviolate'?"
That same testimony mentioned above, passed down as oral teachings (aka "Sacred Tradition").
"It's precisely this kind of silliness which keeps me away from Churches."
Too bad.
"It's not enough that Jesus healed the sick, gave sight to the blind, and raised the dead. That just ain't good enough. Oh no, his mother had to have an 'inviolate womb'."
You read it just backwards. God can have it any way he wants it. If he could raise the dead and pass through walls, he could certainly maintain Mary's vagina in whatever state he wanted it to be.
I'll have to reread my copy of the New Testament. I missed the part where Mary describes the condition of her vagina after Jesus was born.
L