Frank Reagan Did Have a living "significant other".... others actually;

- first, the 35,000 men and women who comprised the NYPD,

- second, the Reagan family, and,

- third, the X,000,000 residents of New York.

Sure, none of these were a physical relationship. But each was every bit as "deep" as any married couple. I personally think the show's producers chose well not having the Selleck role involved in any relationship.