Thats not exactly true. Most of not all the states in the US recognize de facto relationships, or common law marriages.

You get all the benefits of marriage and can even file for divorce and get alimony and/or child support. Between states there are slightly different requirements. For example, in some states you have to be cohabitating 6 months, in other states, it's 2 years, in still others it's 7 years. You have to be a legal adult, and you have to present yourselves publicly and to family as a married couple.