I agree with you but I do think it is valid, from a scientific point of view, to test this hypothesis and to answer the "why" behind changing preferences. It is also possible that overall desire has remained constant but lack of access to contraception, or increased family pressure or other factors contributed to higher birthrates in the past.

I'm not sure what you mean by "even raising the question in the first place is overreach." On an individual and personal level, it may be inappropriate or rude to ask the question. On a government level it is more complicated as I think that while government has no place in interfering with an individual's personal choice, I think there may be utility in understanding changing social trends over time, as a government that is "by the people, for the people" can better represent its constituents and form policies that its constituents want when it understands them, demographically.

But most important to me is that, on a scientific level, it is just data and knowledge. And in my opinion there is never such a thing as "too much knowledge."