A de facto marriage is where a couple has the same rights as a legally married couple by the fact that they are living as a married couple. It is different to a common-law marriage. Quite often de-facto marriages are never official unless there is a reason to test them legally.
However, the point is being missed.
I am suggesting that for genealogical purposes, in terms of family relationships, the production of children, cohabitating, who people live with and when, births, deaths and marriages, these relationships are just as valid as a legal marriage but we don't systematically record them in Wikitree. In fact, we systematically exclude this information. I think the system is poorer for this
When data about these marriages are not systematically recorded historically we lose a lot of genealogical information or there are more barriers put in the way to understanding past family relationships. These types of relationships are not rare by any means and in some historical contexts are quite common.
In trying to define to how this is different from a legal marriage I have unintentionally steered the discussions away from this point.
I may try again in the next few days - perhaps with some clearer examples.
Thank's everyone for their comments thus far and I don't think voting up your own best answer is bad form, if it progresses the discussion (selecting a particular new point that has advanced on others).
I also selected answers that were not my own by this logic and advanced the discussion which is in line with the honour code.
Good night all.