I really believe that a lot more women think about this than you realise.
There's nothing wrong with being attracted to another woman - most of us are at some stage in our lives - either because we want to be like them so much that we blur the line between admiration and adulation, or simply because we find them attractive. Let's face it - women by their very nature are more receptive, emotional, sexy, mysterious etc... and men are just a bit, well, predictable and 2D (which by the way, is fine too!).
Embrace it, but don't panic! If you like girls, you like girls - explain to your friend that you're up for trying anything, and this week/month/year it's women! (are you young enough to think it's a *phase*?)
God, I hope my lovely blokey doesn't read this, LOL!