@whoamiagain I was about to ask the same.
If they were a only-woman family then maybe you could said it was because she had never seen male underwear before, but she had a father and a younger brother so at some point she must had seen it when doing laundry or something.
In another news, d4nm, who knew that Uzaki had the legendary Ultralord Double-Barreled Slingshots? lol.