But, I think all these things go for a long term live in relationship as well. Not everyone in love is allowed to get married, some don't want to, some can't afford to. But, IMO, a good long term live in relationship is just as binding.
There's no guarantee (as anyone who has friends who have been cheated on or has been cheated on themselves knows) that legal marriage
ensures someone is not going to leave you, like how someone else makes love better, get bored or if that person is trustworthy.