it's maybe less of a lesbian thing and more of a perv thing i think.
i know plenty of dykes that would have hit me half way across the world if i asked to sniff their knickers
why? just tell them you're being a gent and testing them if they are ready for the washbasket.
Or tell them not to get there knickers in a twist. (hilarious expression, so British)
The word knickers makes me chuckle, pants, panties, briefs, undies or shorts but not knickers.