The human body is an amazing thing, and as previous posters have said, we are designed to give birth, and with the right positioning, the pelvis opens right up to let the baby out. Have you considered a doula or your own midwife? They can give you ongoing support and help you with positioning during the birth.

Keep in mind that any amount of spontaneous labour is good for the baby. It assists with their breathing, helps with breastfeeding etc etc.

It's great to hear that your OB is encouraging a vaginal birth, but he still needs to hear you out on your concerns. Change if you're not comfortable with how he listens.