"Multiple women have said that the men they know might have done a “bêtise” but didn’t commit rape. In French, a “bêtise” roughly translates to “something stupid.” It’s a word that parents use to scold children for doing silly things, or that a couple might invoke while bickering."
No, a bêtise is when a kid spills her milk or breaks a toy.
In this context I think they use the word as an euphemism for cheating on your spouse, as that's the defense of most of the accused: they thought it was a kink and that Gisèle Pelicot had given her consent.