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That statement could possibly apply to a couple of situations. The situation could either be a couple living together in a common law marriage or be a couple who is separated under an interlocutory (not final) decree of divorce. These situations are described under "Considered Married" on Page 5 of Publication 501 - http://www.irs.gov/pub/irs-pdf/p501.pdf
yes that is obvious, however your statement on the just answer web site seemed to imply that a non married couple who files as joint is thereby required to file either mfs or mfj until an official devorce decree. the question is specifically put as does filing a joint return constitute admission of being married common law or otherwise. Especially if the couple lives in a common law state and has made NO OTHER REPRESENTATION OF MARRIAGE BEFORE THE JOINT FILING.
Filing a joint return does constitute admission of being married (common law or otherwise). If the couple also meets other requirements for proof of a common law marriage in the state that they reside in then these actions demonstrate their agreement to have a marital relationship. Here are some of the requirements for a common law marriage in CO - http://www.state.co.us/gov_dir/leg_dir/olls/PDF/COMMON%20LAW%20MARRIAGE.pdf