This isn't really "staying single," but I think my best financial decision was getting a roommate in a small 2-bedroom apartment in my VHCOL area while I still had law school debt, and keeping that roommate for years and years after the debt was paid off. The percentage of my income spent on rent each month was well under 10%.
(Then I bought a house of my own, which may turn out to be a bad financial decision, but a great lifestyle one. Either way, all the savings from years of living with a roommate mean it's a decision I can easily afford.)