Could you be so kind as to spell out the reasons why you think that it's inappropriate for an 8-year-old girl to watch a lesbian scene?
Please bear with me, as I'm really not trying to make a statement; I simply prefer reasoning over arbitrary opinion. Myself, I don't have much of an opinion on the matter. By default -- since I haven't been convinced otherwise -- I personally think that there's nothing harmful about lesbian sex, just as I think that there's nothing harmful about Sesame Street.
Are you worried that she's going to go out and have lesbian sex with her friends, like some people are worried that playing computer games will make kids go out and kill people? (I'm still not trying to make a statement, by the way. Just take my words for what they are.)