It was explained to me once that [at least in New Hampshire, USA] common law marriage has something to do with the principle of estoppel. Estoppel is the principal that if you do something repeatedly, you would be expected to do it in the future. In the case of common law, if you present yourself as married (for taxes for example) you can't later claim you are single to get out of paying alimony when split up.

Example: If an insurance company pays for flood damage, and they have another flood damage claim, they can't suddenly say they won't pay.

>In many countries, people are eligible to file for common law marriage status after a certain term of living together as spouses. Nothing wrong with that. In others, however, the status of common law marriage is imposed on people living together for a period of time, regardless of whether the people want it or consent to it. This is a violation of human rights as it denies people the right to chose who they are married to. In the country of Colombia, the period is 2 years after which the status of common law marriage is imposed on a couple

This basically makes the choice implicit (live together for 2 years) rather than explicit.

>(...) carrying with it the full range of legal responsibilities of consensual marriage, including property rights.

And this is probably the reasoning - to protect mostly women (as per the customs back in the day), where men lived with them long term, promised them eventual marriage etc, but never did so, thus not giving them any of those legal rights that come with marriage.