Ah, you've just had babies.
Maybe your relationship and career are an egalitarian ideal, but for many if not most women over the years, motherhood impacts their earning power. Going part time, not taking the promotion that means a longer commute, missing out on a better role because your sleep deprived and had nits during the interview :)
That difference widens over time. In 20 years he might outearn you and have a bigger pension. You're relying on goodwill if you think he'd share pension, property, savings etc if you break up. Goodwill is not so strong if someone's run off with someone else/had a mid life crisis/etc.
You can also have a situation like children with additional needs that place huge pressure on a relationship. Unmarried, there's scope for dad to leg it and leave you on benefits with nothing but child support peanuts from him.
The most dramatic situations on here are women who've helped in partner's business, raised kids and kept house, get dumped in 50s with no work history, savings, property or pension (or even state pension entitlement).
I think marriage doesn't glue you together but if you want to do without it, you need to make financial arrangements that are roughly equivalent, which is more expensive and more faff than paying £80 or whatever for a marriage licence. You don't have to refer to yourselves as married or even tell anyone.