Well not sure in America but in the UK. If two couple get married, they get automatic rights stated within the Act of Parliament. I can't remember the name of the act.
This includes stuff like Wills, benefits etc
If not, then you need to apply for a long procedure to obtain those legal rights like a married couple. Its very expensieve this procedure. This depends on what legal rights your applying for.
Plus strangely in the UK, married couples have a higher chance of getting employed O_o
I learnt that in statitics....
EDIT: MY god thats a lot of typos