A cohabiting couple should by no means be legally bound to each other. People live together for a multitude of reasons--financial support being one of them. While people may take the plunge of living together for love initially, financial support may be another driving force that keep the couple together. Also, the government or some other superior force would have to monitor the progression of time in which couples have been cohabiting. The time that this would waste on an insignificant issue would ultimately take away from time spent invested in other topics.
Legal marriage is a choice and not a good one for many people. If there were a rule such as the six month one, then many couples would just call themselves room mates and not say they were partners even if they were. There really is no way to check on this private matter.
I honestly do not think six months is enough time for any couple to consider getting married. Even though that is a decent amount of time to get to know someone, there is a fine line between six months and the rest of your life. I think marriage is still very sacred.
I do not believe that cohabiting couples should automatically become legally married after six months. That is way too soon and many couples break up. It would be a mess! I think it would be acceptable after a couple of years though. Then it could be considered marriage and be okay.
The idea that people should be forced or automatically married is just bizarre. Why would someone even suggest that other than to get some religious ideology in place by the back door.
This is why we need separation of religion and government. This sort of thing fuels the secular movement and with good cause.
Sometimes, couples need more than just six months of cohabiting to determine they want to get married. I believe two or three years should be sufficient to automatically become married under the law. When the day approaches, some couples may decide to split up. The only encumbrance to this process is if one or both people are already legally married yet they are estranged from their spouses.
A couple who is cohabiting should not become automatically married after only six months. That could lead to a lot of couples being in a marriage that they did not want to be a part of. I think it would increase divorce rates even more than they are now and be bad.