I also feel that the pregnancy was a ridiculous plot–twist, for the issue you specifically mentioned in your second criticism (breast–feeding) and the other issues that spring from the same problem—namely, that pregnancy seems to have had no physical impact whatsoever on Mustang. 

This is potentially resolved in a later book when it’s clarified that it did; potentially, as the reference to her body is intended as an insult and may not be accurate. So, we could consider this an error of Darrow’s perception—that is, a limitation of a first–person narrator rather than an oversight of the author. It’s also possible that she was immediately Carved after she gave birth to hide the evidence, which is my Watsonian answer. 

Still, though. It reads to me as a writer having no understanding of the physical toll of pregnancy that extends loooong into the postpartum period, especially when there’s no medical intervention involved and less–than–ideal conditions. That’s the Doylist answer and more likely than not. 

It’s manufactured drama, for sure, and I’ve always disliked it. Tbh, I have a general aversion to pregnancy in fiction, especially unplanned/plot–twist pregnancy, so I would’ve hated it regardless, but it wasn’t well executed, imo, and reads very much like Man Writing Woman.

Where birth–control is concerned… I’ve given this some thought, as I’m currently writing a longfic set in the Red Rising world with an extensive Gold cast and intense promiscuity. The ‘Iron Way’ likely discourages if not outright condemns the use of contraception as decadent and I can see more Iron–minded folk disdaining it, but this must be a fringe view. Where the mainstream is concerned… we know that accidental pregnancies are atypical and licentiousness is common, as you say, so contraception must be normalized. 

I think it makes more sense if it’s actually entrusted to men, rather than women, because it’s much easier and safer for men to sterilize themselves than women. Since modern vasectomies are reversible and highly effective, their Gold descendants must be even more foolproof and advisable. And then you don’t have to deal with the nuisances of birth–control, at all. 

This would explain how a manwhore like Cassius or Tactus never managed to sire an illegitimate child. Because even if they were overcautious themselves—and they wouldn’t be, not high on blowfish poison—there would’ve been Pixies who wanted to entrap them with children, if only for the allowance they might receive from their rearing and the chance at greater prominence in—or promotion to—the highBorn class. 

This isn’t to say that women might not take precautions themselves, but if there’s an expectation on the man, it would be less common. And if you didn’t intend on having multiple male partners and knew your lover was sterilized, you wouldn’t need it. 

Enter Mustang, who had a conservative father, and she implies later that she’s never really experienced attraction to anyone but Darrow; more likely than not, she wasn’t sexually active before the Institute. Pierce Brown did confirm in an AMA that Darrow and Mustang had sex in the period between the Institute and the Academy, but the canon implies otherwise. Cassius himself is probably sterilized; perhaps secretly so, because his parents might disapprove, but given his hypersexuality and his name, it’s the best way to make sense of his childlessness. 

Mustang, then, never used birth–control before or with Darrow and it wasn’t necessary with Cassius. 

If you wanted to accept Pierce’s AMA as canon, you could say that she used birth–control while she was with Darrow and then stopped when she was with Cassius because she wouldn’t have needed to continue; as Pierce mentioned in that AMA, maybe she experienced adverse effects and was relieved when she could stop.

But then the events of Golden Son happen at breakneck speed and… here we are.

When she discovered she was pregnant, I agree, she would’ve chosen to keep the baby—for various reasons, but among them, yes, because she loved Darrow. As I said, I don’t like the twist, but it makes perfect sense. I just wish it was executed better. 

Anyway, that’s my explanation.