Wow, your doctor is making lots of assumptions. I would think that he would at least wait to see if you needed the medication first. You can just say no, and then change your mind later if you think you need it. Is there a reason he is choosing a spinal over an epidural? An epidural will usually stay in a minimum of 6 hours post birth, so would negate the need for other pain medications. Granted a spinal only lasts 2 hours so can make you more mobile but it's not like you're going to be moving around a great deal anyway, and the thing with pethidine is it goes to the baby and can effect lung function, breast attachment etc. especially with long term exposure, like several doses after birth.

How bad is your SPD? I had SPD too, and while it did effect my birth (as in made it very long as it took me a while to adopt the right stabilising posture which was sitting on my tailbone to an angle, plus my baby was posterior) I was still able to have a vaginal birth, and a VBAC at that. Did he spend the time explaining how he believed the risks/benefits of a c-section (both long and short term) outweighed the risks/benefits of a vaginal birth with SPD?

It might be worth asking the questions, looking after a baby and a toddler will be harder after a c-section than a vaginal birth. It was one of the main reasons I pushed so hard for my VBAC because my DD would not have understood the reason I could hold the baby and not her. I'd personally be questioning whether you need the c-section at all. If it's a need then fine, but based upon what you've said, it sounds like the doctor is just trying to go with the more convenient option for him, which will have both short term and long term inconveniences for you.