A releationship can look exactly the same married or unmarried. I can see administrational benefits for the state, but that can be the same if you just register your partner. A simple database is in case someone has legal claims is sufficient here.
I can't see anything an unmarried couple can't have? Do you? Apart from your personal morals?
morality is the foundation of society, to simply abandon morality is what causes decay. A traditionally married couple is different than unmarried and uncommitted partners, and people who take marriage seriously are far more likely to stay in that long term relationship and create a successful environment for children to grow up in.
this can be proven with data, but it also can be proven just anecdotally. Not taking marriage seriously has lead to a more promiscuous and unhappy society, seeing relations as short term gains.
can a relationship look generally the same between an unmarried or married person from a 3rd party observer? yes, but it does not mean that is what is likely to occur or even an accurate observation