Thankfully, that's no longer the case! The miracle of modern medicine means that we can take the DNA of the billionaire man, and the DNA of the billionaire man's wife, combine them in an egg from a second woman, and implant it in a third. The third brings the baby to term, a baby that has none of her disgusting lower-class DNA.

Okay, but now this is mixing up levels of abstraction. The sex ratio is determined based on the fitness gradient of the genes, but this is almost totally independent of the incentives that drive the individual. Most people, let alone billionaires, are actually not consciously motivated to pursue technological strategies to maximize their genetic fitness, even though mutations that induce those drives might theoretically be selected-for. Perhaps that's why most men don't become sperm donors, and most women don't urge their sons to become sperm donors. Billionaires in particular tend to be more concerned about their legacy, estate, and inheritance, and having a large number of heirs divides the pile they've worked so hard to accumulate. But to each their own, I guess?