Definitely a sad song, and perhaps reflects how poly could have provided a less sad outcome, but not about poly.Some of the lyrics to this one make you think maybe she was a first wife....maybe a mistress? Sad song.
From AZ lyrics: Björn Ulvaeus wrote this after he and ABBA's Agnetha Fältskog got divorced. He was unsure about whether she should sing it at all, but she did after he went to the studio and played the song. The song, of course, was very personal for both of them though it's not merely autobiographical but rather a reflection of a divorce. Ulvaeus explained, "A lot of people think it's straight out of reality, but it's not."