Had they continued the status quo, Chan could potentially lay claim to a much larger portion of assets, including a chunk of his $20 billion in Facebook shares, lawyers say.
"In California, people who live together without the benefit of marriage could claim they had an agreement to pool resources and efforts," said Californian lawyer Robert Blevans.
Although they are hard to prove, "those claims can get really ugly."
Blevans cited the case of Anthony Maglica, the founder of the company that makes Maglite flashlights.
In 1994, an Orange County court awarded $84 million to Maglica's girlfriend Claire, who took his name and lived with him for 23 years.
Although an appeals court reversed the award in 1998, she later negotiated a $29 million settlement.
The same logic - avoiding messy court fights - enters into the calculus of a prenuptial agreement.
"One of the primary reasons that wealthy people enter into prenups is to prevent the type of carnage that can come with divorce," said Garrett Dailey, an appellate attorney in Oakland, California, "Better to sort it out in advance."