Listen, God bless the tax preparers for doing their job, but they are not lawyers and I would not necessarily take their "legal" advice.  I am worried here that you could be seen to have committed tax fraud and so I think that you should go and speak with an attorney in your area on this matter as soon as you can.  The general rule about common law marriage - which Oklahoma recognizes is that you have to hold yourselves out to be married.  One of the factors that the court will consider is indeed the filing of a joint tax return.  Please go and speak with some one.  Better safe than sorry, especially while you are on your way to starting a new life. Good luck.