Mc is based, putting her foot down where it matters. And I suspect the girls were deflecting their guilt by trying to demean her afterwards, because they actually realized just how fucked up they are and that they almost did do something that is way too far, as a result of the "overly kind girl that lets ppl walk over her" out of all ppl suddenly telling them a hard no and that it would be unacceptable. That's a harsh wakeup call afterall.
Harsh enough I get why they would need to then subconsciously start deflecting and through that convince themselves that they would never actually go through with it and do that. And that it was all jokes. Because human psychology does require doing so and saving its own ego. Also probably the healthies response in a way, as through reinforcing to themselves that they would of course never do that they also actually become people that would never do that (our brains hate being convinced that we are wrong and correcting itself. So instead they, whenever possible, tend to convince themselves that they were correct all along, and were in fact always aligned with reality).