'What attitude does the Church take toward common law marriages?'
In a country in which the common law marriage is recognized as legal, and the children resulting from such a marriage are recorded as legitimate offspring, the Church will recognize such marriage under the conditions attaching to recognition by the law of the land. However, in a country such as Holland, for instance, where conditions are as presented by President Lyon of the Netherlands Mission, where the children of such unions are registered in the records as 'bastard children and the mother as a prostitute, the father being recorded as unknown,' a couple so living are certainly not living in honorable wedlock, and therefore cannot be recognized by the Church as married people.
With reference to the case of the German sister called by President Kooyman to do local missionary work, who, it has since developed, has been living with a man to whom he is not legally married, obviously the proper course would be for this couple to marry. It appears this cannot be done because the man with whom she is living cannot get a legal divorce from his former wife. In this alternative the woman must of course be immediately released as a missionary.
We are keenly aware of the difficulties that such a situation presents in countries where not only the State but the established church and society wink at such irregularities. Where such relation are of very long standing, particularly if the relation existed at the time the people joined the Church, the Church should urge the legalizing of this union and should refrain from putting its stamp of approval upon such unions by placing in positions persons so living. every effort should be made so to treat such cases that the Church is understood as not sanctioning illicit sexual relations. The Church must always stand for sexual purity.
However, it is our view that persons should not be taken into the Church who are living in such relations, and that every effort should be made to learn before baptism whether such relation exit After persons became members of the Church, illicit sexual relations should no more be tolerated among Church members in Europe than in America, and they must be dealt with there as they are dealt with here. ...
[First Presidency, Letter to Joseph F. Merrill, as quoted in Minutes of the Apostles of The Church of Jesus Christ of Latter-day Saints, 1910-1951, Privately Published, Salt Lake City, Utah 2010]