Nothing, as long as everyone involved is aware and agrees.

Negotiate everything ahead of time, be prepared to also have to enforce the agreements periodically since anyone making that kind of transactional merger of lives is going to be prone to wanting to renegotiate when life throws the unexpected at the couple.

Just be aware that marrying into wealth means you’re going to have to bring more to the table than just being a stay at home wife. Even upper middle class folks that are willing to take a trophy wife damn well expect them to be a trophy, not just a random asshole off the street. The higher the income bracket, the more you’ll need to offer for a transactional relationship.

Think about it. With enough money, a random woman or man can adopt on their own, so they don’t need a spouse for kids. They can hire a surrogate as far as that goes if it just has to be bio kids; then hire nannies. They can throw money around for sex, for housekeeping, even conversation. So you’d have to be damn good at a lot of things to make it more beneficial than just spending directly.

If someone has the kind of income to fully support a partner, and it isn’t a love match, they’re going to expect you to hold up your end of the bargain. If that’s head twice a day every day, and you have allergies that make it impossible during the summer, shit ain’t gonna go well. And there is almost certainly going to be a minimum amount of sexual activity expected. Might not be much, but unless your rich partner is asexual, or otherwise eschews sex, it’s going to be part of the transaction, and it’s just a fact that customers expect prostitutes to perform for the money.

You gotta work that out, what happens if one or both of you get sick, become disabled, change significantly on a physical level, etc. Hammer that shit out before there’s a legal binding.