Non comprehensive, and only vaguely important to me, but being married would mean Healthcare decisions would fall to me automatically, if he dies I'd automatically be assumed to gain his belongings and home rather than very likely have his family contest his will that says the same thing but they could argue that more. They could come in and take over everything were anything to happen to him, and not allow me to see him. No survivor benefits if not married. I'm sure there's a lot more.
Yes, and he has. That doesn't change the fact his family could come in and override decisions for medical care if he were to undergo a coma or the like. It's also more likely they can contest the will as we're not married and have the judge grant them it.
Similar shit happened to my WW2 veteran great uncle in law. His family who he hadn’t been close with in decades tried to take over his life from us but he managed to fight it before he died and was able to move back in the town his wife passed away in because his family moved him for a bit.
This a preventive situation from bad family members, makes sense to be fearful that at some point, someone decides to take legal action and separate you from him, its crazy because as a couple you stay most of the time with him and very likely also share income but as other comments, marrying also implies sharing income and that affects insurance.
In Australia, we have a sort-of equivalent to common law marriage called "de facto" status. It kicks in at the one year mark and carries the exact same negative effects on a disability pension. Actually, I think the effects on welfare and disability payments occurs the second you're living with someone you're in a relationship with.