As a married man of over fifty years I can recall exactly how I felt when I took my marriage vows. I made a commitment. It wasn't to my wife, or my family, or to society at large - it was to myself; a sacred vow to myself.
Had I not become married in the sacred sense of the word, in a church, before an altar (and I certainly wasn't a religious, church-going type of person) I doubt I'd have ever have felt this deep emotional bonding between my ego mind and my own soul.
Marriages can survive a lot more than a defacto relationship in most cases. We know that around 50% of marriages do not last a lifetime...but what are the statistics on 'having a partner' and just living together? I expect they are even worse.
No, I do not think marriage is outdated.