i am bi as well, so can't fully relate, but some lesbians I know don't mind penetration, but don't like it if the dildo is too realistic--which makes sense, since the image of a penis doesn't really turn them on or anything.
I think it's like straight guys being able to enjoy anal because of the prostate stimulation... Even if lesbians aren't attracted to dick, there's still that g-spot to play with.