It's not even that. If the assassin and the roommate made statements to law enforcement that they are indeed lovers, that would matter zero to Roundy.
Roundy believes that because the roommate decided that he wants to identify as a woman and that the murderer, also a male, believes that the roommate is truly a woman simply by declaring it so, then it's therefore a straight (heterosexual) relationship.
Full green sky. The dude is lost forever.