Here we're talking about alimony and how community property works.
The bottom line is raising kids as a stay at home parent is a huge burden. It's entirely unpaid (in money and career). The beneficiaries are both parents. To allow a man (or rich woman, the laws in California are fairly gender neutral here) to accrue the benefits, and disallow the woman any benefits from her sacrifice, well most people consider that to be unfair. And so do the courts.
Surely you mean no leverage? She can abort or put up for adoption, he can do neither, nor "financially" abort (nor prevent abortion if she wants it).
Or consider immigration, border guards, or any other number of more fluid situations. People, generally speaking, respect a married relationship and hold it up at a more privileged level. This is what the marriage equality fight was about, since people usually don't respond well to the clarion call of "but we have a legally binding registered civil relationship in the state of California!". That doesn't roll off the tongue as your partner is literally dying in the ER and you can't see them.