I don't think you have anything to apologize for. In fact, I think you might want to reconsider this group. OK, I haven't been to parties of this sort and I guess this might be standard behavior since others don't seem to find it shocking, but here's my feeling - yes, you knew she was a sub and yes, you met them at a fetish event, so OK, probably the spanking itself shouldn't have been a surprise to you. But for someone who doesn't know you well and doesn't know your preferences to just thrust you into a scene like that, I think it's intrusive and wrong. Furthermore, I think he knew that. I think he was relying on your inexperience and good manners to prevent you from objecting.

I'd be curious to know if anyone else sees it this way.