You do realize that a woman's body goes through a metamorphosis during delivery where their body accommodates the size being pushed through it right? And that their vagina then reverts mostly back over time once the baby is born? And that it is VERY painful? Fisting is nothing like child birth.

 
To be honest a long ago lover of mine and I got very hot and heavy... He basically was fisting me after about an hour of passionate kissing and fingering adding fingers until I was so slippery and wet he had all fingers and his thumb in. It hurt a little but it was very hot and something him and I still talk about... If a girl is into it and there is chemistry anything is possible. And, no, I do not consider this with clients due to the fact that the following day I was very very sore...