I had the same experience. It was a novel where, IIRC, the hero was over 12 inches and therefore the heroine had to position herself in a certain way to allow him to enter her womb. I remember thinking, "At over 12 inches, we're past the uterus. We're into the Fallopian tubes here."
This brings back memories of Valley of the Horses with Jondalar going around ritually deflowering virgins with his Jurassic-sized doomschlong. Even in middle school I was...skeptical.