Re (from Olderwoman
"If there is anyone on this forum who has had (and still has) a successful and happy polyamory longterm relationship that involves a married couple and a single 'extra partner' I would like to hear about it."
[raises hand] MF legally lawfully married couple plus M (me) = an MFM V. We see each other as three equal partners, even though the State sees us as a married couple living with a friend.
What is "successful and happy polyamory longterm?" We've been together eight years and see no reason to dissolve that -- ever. You can draw your own conclusions.
While the word "marriage" does have a questionable past and baggage (though you'd be surprised what kinds of things people could "own" in medieval times -- people owned other people in all kinds of ways), I believe that one important bit of social value marriage retains today is its unspoken impression that "society approves of this union." Whereas the words "civil union" thus far seem to me to tend to imply that "this is a second-class union that wasn't good enough to gain society's real approval." Perhaps those connotations will change in the next several generations, who knows.