Hi there, and thanks for getting in touch! In this case, ‘they are’ was just used in the gender neutral sense, but I agree it could definitely be a little bit more natural!
I am going to change it to -
You are living together, but not dating? So that means they’re your roommate, right?
I think in this case, prolonging the ‘they are’ and adding ‘a’ made the English come off a little like an accusation. I hope this is a better translations that more closely reflects the Japanese.