In recent decades, the stigma of two people living together has been reduced. More people are cohabiting, and it is important that they recognize the difference between cohabiting and marriage. Missouri does not recognize common law marriage, so simply living together in St. Louis will not create such a marriage. If you want to be married in Missouri, you need to comply with all of the statutory requirements to ensure you are legally married.
The one irony of common-law marriage is that in the few states that do recognize it, you can be married simply by living together and holding yourselves out as married. However, should you later decide you no longer wish to live together and want to end your common law marriage, you have to dissolve your marriage just as if you had been married.
If the state considers you "married" via a common-law marriage, you will have the complex problem of divorcing where often one of the parties will insist that there never was a marriage.
This is one of the reasons why the practice has been done away with in most states. It can be a very factually intensive process to dig through two people's lives in an effort to determine if they really intended to live as a married couple or simply roomed together, and many judges find that to be a waste of judicial resources.
For individuals in Missouri, if you fail to marry, you may lose many advantages, from the ability to inherit from your spouse to the loss of health benefits or Social Security benefits. And remember, child support obligations are created at the birth of a child, whether or not the parents are married.
Because obtaining a marriage license and completing the ceremony is relatively easy, if you live together, you should carefully consider the full implications of not being married.
If you do not wish to marry but are in a long-term relationship, you may want to discuss creating some contractual agreements to govern your financial relationship. Of course, getting married would be much easier and cheaper.