To me, the different terms have differing connotations:
Wife/husband/spouse all imply romantic relationship + married.
Significant other implies romantic relationship, official marriage status ambiguous, but essentially equivalent level of commitment.
Boyfriend/girlfriend may be a dating relationship, may be more serious, but definitely unmarried.
Partner is ambiguous on romantic vs other type of relationship, like business, and requires modification or context to clarify. Also ambiguous on marriage status, if applied to romantic relationships.
So partner feels the most ambiguous, which may be a plus if deliberate ambiguity is the goal. It has few connotations of gender, unlike many of the other choices.