Originally Posted by hyperskeptic
. . . or it could just be a kind of progressive distaste for the traditional language of marriage. The terms 'husband' and 'wife' are very, very patriarchal in their origins: husband = manager, man in charge; wife = woman, possibly rooted in a term meaning shame. 'Spouse' seems legalistic and cold.
'Partner' suggests both equality and consent. It does not, in itself, reveal anything of whether the partnership is exclusive or open, or on what terms it is open.
Merriam webster seems to put doubt on this last part. Seems unlikely to be synonym for shame.
The origins in either case are very old. Prior to 12th century.