The line about how "it's like we're married, but we're not fucking" makes me curious about the correlations between a founder's relationship with co-founders and their relationship with romantic partners. You might get some good insight by observing a potential co-founder's behavior toward their romantic partner, or toward potential romantic partners if they're single.
I'm going to get blasted for this, but here goes anyway. I'm willing to bet that the majority of founders are single males who do not understand what marriage is all about. I have not had the experience of being part of a start-up, but I do have 6 years of marriage under my belt. I cannot imagine any circumstance where being someone's co-fonder is in any way like being married to them.
For example, if your co-founder only has 3 months of living expenses saved up, well you've got a deadline to get more money.
Plus in the YC-model startup, you're usually living in very close proximity with your co-founders, so you need to get along with them as roommates as well as partners.