Common law marriage is recognized only in the following states:
Alabama
Colorado
District of Columbia
Georgia (if created before 1/1/97)
Idaho (if created before 1/1/96)
Iowa
Kansas
Montana
New Hampshire (for inheritance purposes only)
Ohio (if created before 10/10/91)
Oklahoma
Pennsylvania (if created before 1/1/05)
Rhode Island
South Carolina
Texas
Utah
Furthermore, contrary to popular belief, a common law marriage is not created when two people simply live together for a certain number of years. In order to have a valid common law marriage, the couple must do all of the following:
* live together for a significant period of time (not defined in any state)
* hold themselves out as a married couple -- typically this means using the same last name, referring to the other as "my husband" or "my wife," and filing a joint tax return, and
* intend to be married.
It is not biblical to declare a couple that has had sexual intercourse, but has not observed any of the other aspects of a marriage covenant, as being married. Scriptures such as 1 Corinthians 7:2 indicate that sex before marriage is immorality. If sexual intercourse causes a couple to become married, it could not be considered immoral, as the couple would be considered married the moment they engaged in sexual intercourse. There is absolutely no biblical basis for an unmarried couple having sex to then declare themselves to be married, and thereby declaring future sexual relations to be moral and God-honoring.
So, what constitutes marriage in God’s eyes? It would seem that the following principles should be followed. (1) As long as the requirements are reasonable and not against the Bible, a couple should seek whatever formal governmental recognition that is available. (2) A couple should follow whatever cultural and familial practices are typically employed to recognize a couple as “officially married.” (3) If possible, a couple should consummate the marriage, fulfilling the physical aspect of the “one flesh” principle.
Both of these put better than I could and courtesy of nolo.com,
Common Law Marriage and gotquestions.org.