That's all well and good, but the reality is that it's usually women who take a career hit when children arrive, and usually women who do the overwhelming majority of the planning and labour of running a family. If a couple are married, that's recognised after a split. If they aren't, it isn't. It seems really unfair that such cases end with the woman essentially completely exploited - or man, in such (rarer) situations.
There's also the point that a child whose parents are married is far, far likelier to remain in the family home after a split, and far less likely to suffer serious poverty. That is not something they control. It's not right that in marriage, most assets follow dependent children, and in cohabitation, all a child is entitled to is bare maintenance.
Personally I think that a couple together for a decade, or a couple who had children and remained together to raise them, should have reciprocal financial obligations. Of course it will occasionally be unfair, as in the example cited, but why should it be unfair a great deal more often, to exclude that occasional situation? And why shouldn't people sign an Intention Not To Create a De Facto, instead, if it's seen as necessary to have a piece of paper? Why is the risk almost always the woman's at the moment?
It's also a mistake to think that de facto arrangement would equate to half the house. At the moment you need to be what they call a "fully entitled spouse" to get a half share if married. If not married, and no kids, and no career hit or greater household contribution, you wouldn't get that much. De factos need not get as much as a spouse. But right now they get almost nothing, even after 20 years and 3 kids, and I really don't know what to say to anyone who thinks that's reasonable. Since when do we think people who've trusted the wrong person deserve punishment for it?