In that context, probably she wanted to get a reaction out of you. It could be that she is a bully, but if she is someone you get along with, or a friend, she may think you're interesting and want to see your reaction.
It sounds silly, but sometimes people pinch or poke people they like, just you see them react.
She probably wasn't expecting you to spank her back, but she was the first to spank, so your retaliation was understandable.
If it continues and she keeps spanking you, and you don't want her too, you should ask her to stop, and if she keeps it up, report her.
He literally said he was spanked on the butt without his consent. No assumptions were made here period.
This is the reason why male sexual assault isn't taken seriously, and why male victims have even less recourse then female victims. Anytime a man is assaulted, it's always brushed off as 'it's not a big deal' or 'it doesn't count cause she's a girl' or 'man up and just take it', often by other men. It's as if consent stops mattering if you have a penis or are male identifying.
Imagine if the genders were reversed and how outraged you would be. As a woman who has had this exact thing done to me before, I know. It's not a matter of how much it hurts to be spanked, it's a matter of losing your agency over your own body and feeling like a piece of meat.