Panties is very American, we Brits wouldn't use it - it sounds too childish. You're sort of stuck, because we also have 'pants' - as distinguishable in our native tone from trousers - but you have eschewed the whole thing. Pants has the same utilitarian function as 'underwear' and can ALMOST be gender neutral..... 'get your pants off!'

As a side note, panties also feels like something a small dog does. One of the really small ones that is in essence a rat on some string. Big dogs pant, small dogs do panties.