Slapping someone on the bare buttocks without their consent in circumstances where they are engaged in sexual contact with others is absolutely capable of leading to a conviction for sexual assault according to Canadian law.
That is true before even taking into account that the actual evidence includes the complainant's testimony that she never asked to be spanked or consented to such contact, that it was hard enough to cause unwanted pain, and it made her expressly ask it to be stopped.
I am one of the first to be willing to debate the merits of what the law currently is versus what it ought to be, and whether we should be criminalizing as much conduct as we currently are, but with respect, how you have articulated yourself in your post is entirely consistent with people who currently get charged and convicted of sexual assault on a fairly regular basis, and many would say is why we do criminalize as much conduct as we do.
Even if it is true that the complainant asked everyone in the room to have sex with her, can you explain how you think that automatically means she was asking / consenting to be physically slapped (on any part of her body for that matter)?
And if you doubt my take on the law, you can review a recent Supreme Court of Canada decision upholding a trial judge's assessment that it made no sense for an accused to claim that a complainant had "out of the blue" asked to be spanked:
https://decisions.scc-csc.ca/scc-csc...20315/index.do