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.
Over thirty years with my partner, not married, same reasons.
Legally you’re a couple, so technically if you each go your own way there’ll be some sort of “divorce” process in relation to assets, but I’ve never looked into it deeply.