To make it easy for you to understand, allow me to provide you with some analogies:
fingernails scratching inside vagina : teeth scratching LB during BBJ = painful, can lead to tears in skin then possible infections
dirt under fingernails : FS with girl who didn't know she just started her period = off-putting, can lead to infections
Before I visit any wgs/sauna girls or go out with civi girls, I always trim my fingernails and toe nails (I don't want to scratch them at all during some vigorous sex) and religiously remove all dirt from under the fingernails. Afterwards as I am making out or doing the foreplay, I would politely ask the girl if she would allow fingers. This is important as it shows you respect her. She may say Yes right away or stop for a few seconds to inspect fingers first.
Even if she turns you down despite having prepared fingers, your chances of getting her to change her mind goes up tremendously if you are daty-ing her to a potential O and she demands you stick something, ie LB or fingers, inside to push her over the edge. She will most likely relent to allow fingers then (I don't always carry condoms, so I have to use my fingers) to have that orgasm.
Women always give us guys shit by accusing us of listening to our small head more often than to our big head, but let me tell you when a woman feels a big orgasm coming and wants it, she will shut down her brain too and listen to her pussy's demands for some fingers.
