I'm convinced that Hinako was originally going to wear simple underwear considering what was already posted but this may just be a coincidence...the fact that Hinako ''says'' this in optional event...it's true that if she wore plain underwear this line would more sense, as it is, it's weird that she's surprised because those 3 girls wear ''such grown-up underwear'' when she also wears underwear that can be considered as such, but plain white underwear is another story and if that were the case, this dialogue would make more sense but again, it may be a coincidence.