Puritan ideals of feminine innocence

I don't think most people here focus on the gender, it might be you who is doing it either for winning an argument by agitation or it reflects your internal state of reference on the topic. This is not related to gender at all in my perspective.

The main argument is difference in mental maturity and compatibility. Younger people, having less experience in life generally will have poorer decision making skills in this regard and are also more easily manipulated by more experienced people. You may oppose such a relation for the same reason you may oppose two people of the same age making kids at the age of 18. Ofcourse if you are their guardian, next natural step is to probe how well they defend their decision and how well informed they are in the potential gains vs losses. Not to reject it out right. Everyone has the right to do whatever the fuck they want after whatever the legal limit is in their country. Doesn't mean you shouldn't discuss downsides of any decision made after 18.

I presume you would be less ok with this if it was 40 vs 16. But any age limit you put between 16 to 19 will be a blanket limit considering the worst possible cases in terms of mental maturity. So being ok for 19 and not for 16 is kind of arbitrary. The only tangible fact here is that 19 is just about the age where your life view really starts to mature where as someone at the age of 40 has had like 20x more life experience in their mature years compared to you. It creates the same kind of power imbalance that a teacher student has. This is also not the same as 30 vs 50. Here the difference in mature life experience reduces to about 3x almost an order of magnitude less imbalanced.