It's a fictional age in a fictional world.
Am I? She was his right-hand woman, not some random girl. I don't think saying it was insensitive is untrue (her reaction is proof enough), and I don't think his attitude in that particular context was a great display of gender equality.
I don't think you understand what "insensitive" means. The definition of the word is "deficient in human sensibility, acuteness of feeling, or consideration." Having his second-in-command keep a wounded enemy warm at night "because it's a woman's job" shows a pretty severe lack of consideration for that person's feelings. There really isn't much to debate here. And I didn't say anything about testing her loyalty either, on the contrary... I meant that her presence could have been a way to ensure Guts' safety.
you know I never considered this before you make a good point. I never understood why Griffith would order her in the high handed way that he did to warm Guts using such a reason as that. And since Judo told Guts Griffith's exact words when he ordered her to warm him, it suggests he ordered her her right in front of the other men (or at the very least in front of the commanders of the hawk/falcons) which must have embarrassed her. I always kind of got annoyed with Casca's reaction (punching Guts in his wounded stomach/chest), but taking this in consideration she had a good reason to be pissed.
There is no doubt that Guts would have tried to catch any member of the Band of the Falcon if they'd been falling off a cliff. And taking care of Casca as a woman versus taking care of a man was, if anything, a hindrance for Guts. As warriors, the guys likely had no problem seeing each other naked, that's part of life in such a setting.
However, if there is one point that could be contentious, it's the fact he warmed her up with his own body. Guts had a distaste for physical contact at the time because of what had happened with Donovan, so it might be argued that he would have been reluctant to do it, even though in the spur of the moment I'm not sure that would have come into play.
I did not think he was that much bothered by having to touch Casca as he was with the fact that he was he was self conscience/put-out over having to undress some one who already hated him and has a history of reacting hostilely toward him and that that his stripping her would cause a volatile reaction from her...and he was right. Guts expression over contemplating
what to do for Casca:

but I did not consider his distaste for physical touch, only because I thought facial expression during those scenes came across to me on the lighter side then it being personaly unpleasant for Guts.
Also I agree Guts wouldwould have helped any one out in the same situation as Casca. It's not like after dragging who ever out of the river and finding out they had a fever he'd be like "oh your sick? Hope you make it through the night buddy, I'm climbing back up the cliff..."
The only thing I can't buy is that he would have actually warmed another man up with his own body like he did for Casca. He certainly would not have had a fellow hawk perched on his lap at the mouth of the cave like he did Casca.