""A very common story is the girl who has sex believing the man has made a long-term monogamous commitment to her, but she finds he hasn't -- and she finds this extremely painful emotionally."

This is also true for adult wimminz and menz."

True.
Sex is special because man is K-strategist.
While few other mammals are fathers, many others, such as many birds, are also fathers. And some animals, like elephants, are grandmothers.
But is anyone besides man both a father and a grandmother? Is anyone besides man a grandfather?

A man is committed to long term raising of children the moment they go to bed.
Actually, the emotional and social investment to rearing children and long term common life often starts before actual getting to bed, when dating gets serious. And therefore relationship breakdown potentially means lost and wasted investment - emotional and social.

"And yet, in our cult of the magical natal anniversary number, those above 21 18 16 whatever are expected to "tough it out" and those below are expected to be denied all agency for fear that they may come to regret something decades later."

Does not mean that the results magically and suddenly change.
A large proportion of adults make mistakes in breeding.
But not breeding is also a mistake.
And time is limited.
If 51 % of 18 year old girls make right choice of partner and 49 % are wrong, while 49 % of 17 year old girls make right choice of partner and 51 % are wrong, that´s a tiny quantitative difference. Yet the meaning of Age of Consent at 18 is that 49 % of 18 year olds are permitted to make awful mistakes in order to allow 51 % to find happiness, while 49 % of 17 year olds are forbidden to pursue happiness in order to protect the 51 % who would be wrong.

"Now we do have laws concerning fraud and breach of promise."

The problem is that most jurisdictions don´t. Most jurisdictions stay out of bedroom - meaning they have few options besides blanket permission "all is fair in love between consenting adults" and blanket ban "children cannot consent".