In several countries, the government imposes a legal status of spouse-hood upon people living together for a certain number of days, regardless of their will, after which it treats them as it does a married couple (in marriage people by choice obtain the legal status of spouses).
Since it is done without the will of the people involved -- often against their will -- it is hard to understand how this isn't an obvious violation of human rights, as well as civil (Constitutional or Charter) rights in nations that have those protections against the government.
Against human rights
Most countries are signatories of the UN Declaration of Human Rights. The U.S. and Canada were among the 48 countries first to sign the document in 1948.
Article 3 contains protection for "fundamental human rights" - life, liberty and security of person. It's hard to see how choice of marriage wouldn't fall under fundamental rights of life and liberty. The document also specifically treats marriage in article 16, where it states, "Marriage shall be entered into only with the free and full consent of the intending spouses."
Against civil rights
The Fifth Amendment of the Constitution (U.S.) and Section 7 of the Charter (Canada) protect the right to individual autonomy and personal legal rights from actions of the government.
The language used is "life, liberty, or property" in the Constitution and "life, liberty and security of person" in the Charter.
The right violated by a government-imposed involuntary state or marriage would be that of liberty, which has in law been interpreted to include the power to make important personal choices. Marriage (by any name, and including a legally-binding state of shared assets), is an important personal choice, and therefore governments with these restrictions cannot compel it without infringing civil rights.
There is much discussion currently about forced arranged marriages as violations of individual rights, but not about forced common law marriage. There are similarities and distinctions. Forced arranged marriages involve both the choice of a partner by someone else than the individual married AND the imposition of an involuntary state of marriage upon them. Forced common law marriages have an imposed involuntary state of marriage to a partner chosen by the individual for a relationship, although not for marriage.
NOTE: Legal scholars make a distinction when it comes to "family law" in that civil protections may protect against the government but not against family members. Therefore, Section 7, for example, may not protect a "family member" from another where it would protect an "individual" from the government, some have argued. This doesn't affect the discussion at hand, though, because it would have to be applied ex post facto to the imposition of "family member" status on individuals.
NOTE 2: It doesn't seem relevant that people bound as spouses this way are not called by government "married" since they are treated as though they were married. In other words, there is no difference except the word.
I post this mostly to find counter-arguments. Common law marriage is a widely practiced institution (although often enough unpopular with people), so maybe I'm missing something?
Some people on discussion forums were doubting that this even happened. Article on Canadian example: http://www.cbc.ca/news/canada/british-columbia/common-law-couples-as-good-as-married-in-b-c-1.1413551