I think you've had good responses here.
My 12 year old has mentioned that she is is going to marry a woman when she grows up, and as a) she is 12 and marriage is a long way off for her and b) it doesn't matter anyway, I just say "yes, that sounds nice." just as I would say "yes, that sounds nice" if she said she was going to marry a man when she grows up. For what it's worth, I don't think she is gay, just that marrying a girl probably seems like a nice thing to do, which it objectively is!
That's fun what @JensonsAcolyte says above, because in my admittedly limited experience of the term, I have never known a (self declared) pansexual who wasn't in a long term committed relationship with a member of the opposite sex, and the only couple I've ever known who said they were in a "queer relationship" were a man and woman.
Fundamentally, your daughter is not going to accidentally end up in a long term committed relationship with a woman by mistake because she was confused at 11. Either she grows up bi or she doesn't.
I definitely wouldn't talk about "coming out" as bi or anything other than "that's nice, dear" until/unless she actually brings home an actual girlfriend, though. It can be difficult to walk something back when you change your mind, and life is mortifying enough as a teen without having to come out as straight to your parents because of a throwaway comment at 11. 