It's not a moral issue, it's a practical one and an emotional one.
The emotional aspect I dealt with upthread - if one partner says "it's just a meaningless piece of paper" and the other one says "but it would mean a lot to me", then there comes a point where the anti-marriage partner has to decide what matters more to them - the feelings of the partner they profess to love, or some sort of commitment to being anti... well, anti what, precisely?
The pragmatic aspect depends on age and stage of life.
If I were a woman in my twenties, I would want marriage to provide security against loss of earnings due to kyboshed career progression following maternity leave/ lower salary due to going part-time, and also (depending on the value of the family house) protection against losing my house through inheritance tax were my partner to step under a bus.
In the position I'm in now - single parent, in my 50s, good job, reasonable earnings, decent pension, house almost paid off - I would not want marriage, because I wouldn't want a partner having a claim on my pension in the event of a split, or having a claim on my house which I intend to leave to DC when I eventually pop my clogs.
Nothing about the morality or otherwise of marriage, purely about the practical consequences.