Significant Other is a term that can be used when person one is dating is non-binary in place of girlfriend or boyfriend. The term can be used when one is dating a non-binary person, replacing the “girl” in girlfriend and the “boy” which may make some uncomfortable.
Similar terms include enbyfriend, joyfriend, theyfriend, datemate/datefriend, partner, shipmate, and loveperson. Also see Gender-Neutral Terms.