imo i don't see it as childish at least, and it makes sense that casca just did not see guts and griffith's exceptionally sweaty behaviour coming.

like, we the audience know there was no reality in which griffith was going to be cool with guts leaving - it was a direct threat to him both on a human fear of loss level, and a challenge to his control freak / leader of men angles.

casca was imo perhaps the most socially intimate with griffith (remember, he cries in front of her and admits to a lot of sad sex shit he felt he had to do for his goals), and thus was sort of blind to how serious shit would get and anticipated a more neutral, familial solution. imo casca probably always saw griffith as a lot more compassionate than he really was, like with the story she tells guts about him saving her by throwing her a sword and then taking her in. to us as normal functional people with like friends and hot rockin tits we know that what griffith did there was cold and weird - instead of saving someone from being raped you toss them a knife and tell them to do it themselves? and we know that griffith would've just left if she failed to defend herself, too, but casca doesn't see that, she sees a softboy who saved her.

and remember this is the point in the story where everybody has socially slipped from griffith's circle b/c he's hanging out with the nobles and shit instead. he's changing and the band doesn't really have a firm grasp on where he's at any more. no one sees the weird sex / rape moment (fans disagree on consent here iirc) with charlotte coming, and absolutely no one sees the behelit moment coming.