I haven't had this issue about spanking, but something very similar happened about TV a couple of years ago. I was at a playdate through a moms group, so I kind of knew the other moms but not really, and near the beginning another mom asked me directly if I ever let my son watch TV. I don't, so I said so, although I wouldn't bring it up because I know it can be so emotional.
Throughout the WHOLE rest of the playdate, the other moms kept bringing it up, and how useful they thought certain videos could be, or how they used to say "no TV" but then they realized that was unreasonable, or how I could only do it because I had one child, and as a mother of 3 there was no way..... on and on and on. If it had just been one conversation, I would have left, but it just kept being a sentence here or there peppered in. I felt very cornered.
What helped me, and I hope could help you, is realizing that they are only so defensive about it because they had doubts that they were right. My being different brought up fears that they were wrong, so they had to reiterate their position over and over, and list all their reasons. It wasn't really for me, it was for themselves; I just happened to be the catalyst.