I was thinking in the shower the other day. And this is a two-parter, so buckle up.
So, first: if you think about all your friends, your good friends, you know whether or not you would want to (be able to?) live with them long-term. Right? The point isn't really whether you would or wouldn't be willing to be roommates with any particular friend--I think everyone knows people who are great friends that you just couldn't live together. No worries. The point is, you have a definite answer.
Now, here's the thing. You can give that answer because you know a lot about those friends, about how the two of you interact. But. What if we did that same thought experiment with those same friends' spouses? I guess you could think of it as "would you want to be married to that person?" but I'm thinking of it more in equivalent terms, leaving aside sexual compatibility (or interest).
Maybe it says something about my adult social life, but I'm guessing that most people would struggle to even answer that question. Most of my best friends, I don't really know their spouses well enough to really answer that except maybe in a lukewarm way ("yeah, I guess").
And there's something kind of weird about that. You know what I'm saying? Like, here are these people that you care about, people that you know well, but you don't know this other person who is almost certainly the most important person in their life. Interesting, isn't it?