What I don’t understand though, is if the female dog has a “receiving” appendage or structure, that assists in the the tie up, then how did the tie up occur with a human where there is no such structure?
The functional equivalent is the anus. If the dog's penis swells up bigger than a two year old's bum can stretch, they are effectively locked together.
Do I have to draw you a picture?
Actually, it is the sphincter in the vagina of the female dog that entraps the engorged and swollen corona of the dog’s penis that causes the tie up in dog mating. The female has control in the mating ritual. The bulbous penile bulb rostral to the corona would prevent deep intromission into the child’s anus. The story just doesn’t hold water IMO. I may be proven incorrect but the possibility of the situation and my experience with animals makes me skeptical, really skeptical.