Before people get too up in arms about agendas and stuff.
The story has already been fairly in your face about things like racism & sexual abuse, it was just using demi-humans in place of ethnic minorities and that was okay. Here we are now, with 2 of the main characters that were a part of that plot line. And we're tackling trans issues.
I presume that this is meant to cement the humans from earth as forward thinking, open minded individuals more than it's meant as a comment on any specific issue, it's just using the trans issue as a vehicle to further drive home the fact that he isn't a human, like the ones they are used too from their world.