Why is a person described as NA?

Could someone please explain to me why Men and couples have become NA and what it stands for? According to all the references I can find, it is one of several abbreviations for Not Applicable and, in chemistry, Sodium. I guess you can rule out Sodium. Not Applicable is really not logical when it comes to describing your sex is it? It's also very confusing when a couple is described as being Woman and NA. Does that indicate that the Woman's partner is a Man or a Woman .... or God Know's What? Come on, let's use proper English (or Americanese if you come from across the Pond) that is understandable to everyone!