Content warning for lots of sexism ahead
In English*, "girl" can have negative connotations in certain contexts. These connotations are far more negative than "boy" or even "kid"; both of those lean more towards endearment than denigrating.
The guard is leaning heavily into these connotations.
“They went on hunger strike. Even the pregnant ones. Wrote up a list of demands—there were like twenty different species of plants on it!” He snorted, swiping his hand over a DNA-reader.
The guard snorts, indicating derision. He sees the demands of the prisoners as illogical, impossible, inane. Only "girls" would come up with such a silly plan. They don't understand the real world, don't understand that their dreams and fancies will not be given.
“The warden ordered the medical staff to force-feed the girls. They lost their damn minds over that,” he shook his head.
The “girls” are irrational and lose “their damn minds” over the consequences of their actions. That's what the guard is saying, anyways. He shakes his head at their irrationality.
Or maybe he thought he had to punish them for the protest. To control them.
"Girls" can't take care of themselves. They must be watched, controlled, herded, to make sure they follow the rules. They must be punished for their transgressions to maintain a society for all of us logical, useful, non-girls.
The very fact that they're in this ward in the first place indicates that they're irrational:
I had been taught from elementary school to look with horror on the years before Turner vs. Alabama. To feel superior to all those lawless, Northern states where the genocide of the unborn continued unabated.
Young children are taught to despise those who get or try to get an abortion. Therefore anyone who does try to get, or help to get, an abortion is unworthy of any adult title. They're only "girls", only silly little kids who can't follow the rules.
Not every use of "girl" is loaded with these negative connotations. But I believe that it's clear, in this situation, this guard is aiming for the negative connotations of "girl". There's some transphobic/misogynistic sentiments, sure, but what I mainly get from this passage is a constant attitude of contempt as the guard belittles the prisoners.
The negative connotations of "girl" are mostly sourced from my lived experience as a "girl", and a native English speaker who reads lots of books. However, the same sentiment can be seen in ideas such as calling girls “bossy” for trying to assert themselves (so this behavior is wrong/unnatural), “crying like a girl” (girls are emotional, weak), and generally “acting like a little girl” to mean being silly/irrational.
* and probably other languages but I only know English and that's what the story in written in anyhow