Maybe pre-nup has a different meaning where you live. I'm in the US, and here a pre-nup is an agreement that defines custom rules for a marriage and divorce that are override, add to, and/or clarify the rules of the default marital contract. So, not everyone has one.

Wikipedia defines it well:

> Couples enter into a written prenuptial agreement to supersede many of the default marital laws that would otherwise apply in the event of divorce, such as the laws that govern the division of property, retirement benefits, savings, and the right to seek alimony (spousal support) with agreed-upon terms that provide certainty and clarify their marital rights.

https://en.m.wikipedia.org/wiki/Prenuptial_agreement

What places have mandatory pre-nuptial agreements?

Everywhere? The point of an explicit pre-nuptial is to opt out of what would happen by default. It's opt-out, not opt-in. If you don't have a pre-nup, then you get the standard pre-nup terms which have usually been established over a long period of time, with detailed rules about homes, children, property etc.

Do you think that’s what GGP meant (or others would most naturally read) in the phrase “I’d be in favor of mandatory pre-nups”? I do not.

Well, that's why I asked my question, ("What are you proposing that's different from that?") because I don't know what he means by mandatory pre-nups, because we already have a mandatory pre-nuptial agreement, and indeed it sometimes kicks in literally before any nuptials.

Clearly they intend something else than a mandatory agreement you sign up for before marraige, because that exists, but they're not really providing any details on how it would differ?