The downside, in America at least, is not just losing some of your wealth. It's becoming truly impoverished and becoming homeless. Remember, America is a country where you can easily go from solidly middle-class to utterly bankrupt because of one medical problem. The stakes couldn't be much higher, unless they institute a death penalty for divorcing.

>I've been with my partner for over a decade, and we're not "married".

That kind of long-term trial isn't feasible once you're past your 20s, if you (or the woman) wants to have kids.