I think it is because a "marriage mate" sounds like something someone else (read: Jehooby) puts together. You "mate" something. You look for the mate to your shoe. It is a very external sounding description.
Etc.
A "spouse" is more personal -like it is you and your husband, no one else allowed. But the WT knows that is not how they want it framed. They need to insert themselves or Jehooby in there so they can control things.
I think my mom used to talk about not dating til you were looking for a "marriage mate" UGH. It is so clinical sounding. Shudder. Bleah...
The way the WTS deals with "marriage mates" usually ends up turning it into "CHECKMATE" Ha ha ha.