It's true, if Grace did have a partner or children or any other family Stratt absolutely would have used those people as a reason for Grace to go. She would have gone harder into how people would die if he didn't. Her only goal was to get him on that spacecraft by any means necessary.
But that's kinda the point, isn't it? It's not that Grace had no one at home was her base logic for why she picked him to go to space. Grace was going to space and she needed to make a logical argument. And the argument she chose was "you don't have family, therefore you have no reason to not go."
Because that's what matters, that's the main thing that would stop someone, surely. Grace is a charismatic guy who made several friends even with her in his time working there. Grace has multiple years of students and a school worth of staff who know him, he has neighbours and friends and a community he participates in. He has a career and hobbies and a home and possessions. But none of those matter enough to stop someone from going to space. They can just up end his life and move him to a remote base far from home where he isn't allowed to talk to anyone he used to know, isn't allowed to go anywhere, it's no big deal to send him a bit further.
He didn't have any relationships important enough in the eyes of their society for it to have legal ties that could complicate things, and that's what mattered. A social construct so deeply rooted in their society that those relationships are written in legal contracts and government papers, those are the ones that are designated to matter. They matter so much that certain rights are withheld from anyone outside those relationships. Grace could be as happy as a clam but as far as any government and person living in the society formed by that government is concerned he was a lonely man with no reason to not sacrifice himself, and so that becomes the pitch.
The pitch could have been anything, but what it was is based in a heteronormative amatonormative understanding of how people and society should work. And it doesn't matter whether Grace was aspec at that point, what matters is he didn't meet the standards for a person who has sufficiently participated in society, and should thus when made aware be willing to rectify this at the cost of his life.
It just so happens that an aroace person who inherently has a complex relationship with these systems by their nature is painfully familiar with that experience.
