Just to set the context, I am not saying, "It has never happened before so it cannot happen in the future". I am refuting the contention that, "It has happened in the past so it can happen again the future". If it hasn't happened in the past, that removes one proof to how it is possible. But it leaves open that it might happen as a new thing that has never happened before. As you note, our society is full of new things that have never happened before, and that includes many good things having to do with liberation of individuals. That includes female choice and child choice in many areas.

It's worth distinguishing two cases. One is a choice for marriage partner, or maybe more precisely "relationship assumed to be permanent unless there is good reason to end it and that might involve children". The other is sex partner of a relationship that is transient. It include the one-night stand (or the one-minute stand).

WEIRD societies are moving away from the idea of girls getting married young, as we want them to get an education and consider life options and make such a weighty decision when they are 18 or older. That has to be a good thing overall. It clashes with the child liberation idea that the girl ought to be able to choose that course. We certainly do let the 14-year-old choose motherhood if she gets pregnant. It's an uncomfortable situation, but sometimes we do let the 20-year-old man marry her and live as a father to the baby, and other times we lock him up. It's a classic trade-off of deterrence on the one hand and best-situation-after-the-fact on the other. Early marriage has happened in history, at times no doubt with the girl enthusiastic about it. But with one exception, I don't see GCers wanting to bring back that situation.

Today, just about all relationships involving young people in general and teens in particular are assumed to be transient. We can make a further distinction between girls who are well into puberty, who have strong sex drive and romantic inclinations. I can imagine society changing to allow relationships with parental consent -- it surely happens informally today quite a bit (and more in the 1970s in certain subcultures). I'll leave as a big, different topic whether that would be a good idea or not (I tend towards "no"), but I can imagine society changing to allow that.

The other case is the 9-year-old girl involved in the transient sexual relationship. It is far less likely that acceptance will ever happen, which I could summarize for practical purposes as "never going to happen". I have my doubts about any GC anecdotes of girls that age just so eager to do sexual things with a man if only it were legal, independent of wanting to make him happy. Add further whether she would deeply regret it, even if society didn't pressure her to. If that course is available, it is exceedingly rare. It would be good social policy to prohibit it if there are a thousand times as many cases where the girl's consent would be misperceived by a man eager to see it a certain way, not freely chosen for the right reasons, and later deeply regretted. If you put in parental consent, there could be a hundred times as many cases where it is some sort of quid pro quo from their point of view with the man and not the girl's actual choice.

But in this we're getting off the main topic. I do not think you can find any past historical examples that match what is being proposed for the 9-year-old girl.