I am married, obn, and have several good friends who live together without being married. You asked what the advantages and disadvantages were:
Not everyone's reasons are exactly the same but in general the reasons I hear a lot are...
In favour of not getting married:
-it's nobody's business but our own, and certainly not the government's.
-weddings are ridiculously expensive and we'd rather spend our money on useful stuff like food, clothes, housing...
-it's only a piece of paper and we don't need that to prove we love each other
-it's patriarchal and old fashioned and not relevant to modern life.
In favour of getting married:
-we want the security of knowing our pensions, parenting rights and responsibilities, inheritance etc are sorted out, and a marriage certificate is a one-stop shop for all of this.
-always wanted a big do
-didn't want to upset/disappoint my/our parents (it's the proper thing to do)
-religious/moral reasons (don't want to 'live in sin')
But my own reasons don't quite fit on this list, to be honest. I was considering moving abroad with my kids and new partner and we wanted to make a clear statement to my family that this was not on a whim.