Not with any of your points about her age, though. It's true of course that 12 would be unusually young for sexual activity, but it's a normal enough age to experience sexual attraction, especially considering how girls are entering puberty earlier than a few generations ago. More to the point, though, while the term may be 'sexual orientation', it refers more broadly to romantic interests. And it's perfectly normal even for children of considerably lower ages to experience romantic attraction. So it's really not that absurd for a child to consider herself a lesbian even before puberty. There may be cases in which that shifts during or after puberty, but that doesn't mean the child's environment should completely ignore it.

So unsurprisingly, I strongly disagree that sexual orientation, just because it happens to contain the word 'sex', should be a taboo subject for children. Even if you accept that sex itself should be a taboo subject for 12 year olds, which I don't but will pretend for the sake of the argument, you can and should still discuss the romantic aspects of 'sexual' orientation with kids of that age.

Lastly on the more general point of what the opinion of a 12 year old is worth (note: I'm not saying any of the following necessarily applies to this particular 12 year old), I'm not telling you anything new by pointing out how frequently children of that age have historically entered marriages, taken an active role in ruling states or done other things that are rather more demanding than giving a speech about their sexual orientation. That's not to say they should be treated as adults, of course they are still lacking in experience and they aren't fully grown either physically or mentally, but 'absolutely no point of view of any possible interest of value to the adult world' is ridiculous.

By way of illustration, Catholic canon law has 7 years old as the 'age of reason' which is the minimum requirement for things like confirmation and confession. I dare say it's very exceptional to be confirmed at 7, but I for one was confirmed in sixth grade, the customary age around here, when most of my classmates were 11 or 12, and I was 10. Similarly, bar mitzvahs and bat miztvahs are done at ages 12 or 13. I think it's safe to say that Western religion generally does attach value to the opinion of a 12 year old.