No great surprise: Gavia and Ardent have become 'contaminated' by going to Earth and are exiled for life. All for daring to be something other than what the Praeses have decided for them (ironically, in Gavia's case, to protect them).
In a relatively closed system like a space habitat, unknown foreign elements are a legitimate risk to the entire system. It isn't a matter of prejudice or arbitrary authority, it is a serious security concern.
Yes too many sci-fi series gloss over quarantines which would be a necessary procedure when traveling to worlds with potentially dangerous microbes, diseases, and viruses. Hopefully the decontamination process isn't too painful.
There's a nice variation on that idea in
one of the The Expanse-novels - though the first actual alien/human 'infection' is a water-based microbe (smth. like an
Euglena) discovering its penchant for the glassy body of the human eye, rendering the human in question blind in the process due to refraction off the bodies of the microbes, so it's less an actual infection than merely a physical intrusion (the microbe doesn't
do anything to the tissues in the eye, it just multiplies happily & exponentially in an environment sans predators). An alien biosphere is unlikely to run on terrestrial DNA (so it wouldn't know how to use us as replicators), unlikely even to use the same
chemistry - but at the same time, if an alien microbe can tolerate the environment in our bodies, from its POV we're large, pristine habitats without any of selection pressures that normally keep it in check.
The idea is that most likely, infecting us would be like trying to eat tar, or a brick of frozen gasoline - but some form of life is certain to discover a use for almost
anything. Maybe we'd end up being the fuel for the barbecue rather than the steak.