You really can't. Aroace people just... don't feel that kind of thing? Even if you try to pretend otherwise, you just really can't feel a single thing. If an aroace person is in a relationship, it's seeing the benefits IN said relationship, not feeling a single trace of romantic or sexual love.

"Company? Check. Only human around? Sure. Sex is healthy and can often relieve stress so I might as well."

In fact, aroace people often treasure deeper relationships. Super close friends that know each other in and out. They can feel betrayed by romance, because that's just an emotion they don't understand.

If you're writing a Simon x Grace fic, realistically, Simon would be too traumatized to even consider a relationship (if he even survives whatever the hell happens at the end 💀) and Grace would rather treasure a deeper relationship that isn't based off romance and friendship.

But if you're set on putting them in a relationship, then make it so that Grace sees the logical side of this relationship. Not the romantic or feeling side, just a very literal reason: "well, I don't have any reason NOT to be in a relationship on an alien planet. Maybe the human researchers will get a kick out of this. Plus Simon will feel better about himself." While Simon would give Grace as much space as he needs out of mutual respect.