Here in CA, if you stay in the committed relationship (as in, renting or buying a house together, having children, several years together) for several years, the court WILL have a say in redistributing your assets anyway if the two of you break up. So not getting married is not an advantage unless you take steps to make sure it doesn’t appear to be a committed relationship.
Not getting married might not be an advantage but when all it takes to end up in divorced is a $1500 retainer and an argument, I can see why people without all that much money to burn just don’t bother with getting married.