In their entry for “domestic partner”, the American Heritage dictionary notes that, "Two other terms, significant other and domestic partner, however, have seen widespread use since at least 1985 as all-purpose words for describing a spouse or a lover. Over 75 percent of Usage Panelists feel these terms can be applied to members of either heterosexual or homosexual couples. Perhaps because the noun use of other in reference to people is not very common, significant other has not been as widely favored as domestic partner. The latter is used by an increasing number of companies and organizations in drafting benefits plans that include all members of such nontraditional families. " I’m not saying I necessarily agree with that. The link follows: http://www.bartleby.com/61/26/D0332600.html Emphasis added.
The term was originally coined by psychiatrist/psychotherapist Harry Stack Sullivan, who died in the late 1940s, and was used to denote someone who serves as a social reference point in a person’s development. This could include parents, teachers, etc., if I recall correctly. No idea how it got corrupted into its present-day meaning.