Auberginefleur- a kimono enthusiast - is refering to the underwear for 'real' yukata, worn at firework displays, matsuri, and during the summer months.

These are not the same as the yukata worn in inns.

Whilst I could happily go into a long post about suitable underwear for yukata (and it would be a very long one) there is no need here.

Inn style yukata robes:

As these do flap open if walked in with long strides, some underwear is very much advised.

Wear any kind of underpants you wish, boxer shorts are fine. A lady conscious of her modesty might like to take a knee-length petticoat, but this isn't necessary, if they do feel unsure, a lady only needs to take smaller steps, optimally with the toes pointing inwards, and the yukata edges will remain together.

Again, if you prefer to wear something on your chest, a vest or v-neck t-shirt will do.

And, as any guide to inn etiquette will tell you, wrap the edge by your right hand over your torso first, then wrap the side by your left hand over this before securing with the belt around your waist. (right over left is how the dead are dressed)