I don't know.. she's the female Sheldon and like him she's socially totally inept or rather very factual and scientific.
She's reads about how a slumber party is supposed to work and fails to realize that the last part with the lesbianism is more of a male teenager fantasy (it said so on the "manual"!
) so she went through with it.
So she was quite in character.. asked about her sexual preferences she'd supposedly cite some neurochemical facts about sexuality and that to a brain it's all the same no matter where the stimulus is coming from... it made for a hilarious Penny reaction though.. too bad it was behind closed doors.
And isn't Bernadette the cutest woman ever.. i find her more attractive than Penny!