Married people are much more financially secure and thus capable of investing in the future. They can not only share expenses, but they amortize risks and can share trade-offs. This is borne out statistically (https://www.city-journal.org/html/why-marriage-good-you-1200...), but also is painfully obvious on inspection. If you’re going to start a business with someone, would you rather do it with a Partnership Agreement in place, or an unwritten “we’ll work together long as it’s convenient for both of us” understanding?