I’m 36 and have been with my current girlfriend for almost 5 years now - my longest relationship yet. We live together & everything else couply, just not married. I see marriage as unnecessary pressure that has a tendency to put strain on the relationship. Right now if we’re happy together we stay, if we’re not we split. Simple. Marriage complicates it.
In addition to that, having kids, crossing international borders and moving to different countries are also things where marriage gives benefits.
And in some jurisdictions, there can be very significant tax advantages.
Which is why there was always a big fight for marriage equality.
But if you don’t want all these legal benefits, then co-habiting is fine.