Maybe.
However, I believe that some states automatically declare a couple "legally married" if they have a "let's just live together for now" scenario for a few years.
Seriously, I think that some states here in the southeast treat a "shack up" as a marriage after 3 years.
Is that true?
Only a few states still allow common law marriages to be formed, and the couple has to hold themselves out as married. There is no time limits, a couple living together for 20 years may not be considered to be married, while a couple living together for a year who visits D.C. for instance, stays in a hotel as "Mr. and Mrs. X" may be considered married.
Some states have "common-law" marriage statutes, but I'm not sure what the legal status of that institution is. I don't know that it begets the same fiduciary obligations as a certified marriage, for example.
Yeah, it's called a common-law marriage and in certain states, women can demand alimony if you live with them for a while and then break up.