I’m in my 40’s and have never been married but did have a lengthy cohabitation. I also had a lengthy relationship that did not involve cohabitation.
If I ever get that serious again, I believe I would forego the cohabitation again.
To me, it’s way too much of a sacrifice without getting “guarantees”. Too much privilege without the responsibility. Like someone upthread mentioned, when you combine a household without anything legally making it a household sometimes the headaches just aren’t worth it. There’s always the “yours” and “mine” to keep track of, and there’s never a “ours”.
As an example of how I feel, I had this conversation with, um, “Cohab” once:
Cohab: “So, what am I to lorinado jr?”
Me: “What do you mean?”
Cohab: “Well, ‘my girlfriend’s son’ is so awkward. Surely there’s a name for that relationship.”
Me: “Yes, there is. It’s ‘I’m too insecure to actually marry her and call him my stepson but I still want the fancy privilege of forging a formally named relationship with her kid to denote that I have a responsibility to go along with the privilege even though that responsibility is one I’ve chosen to avoid.’ But since that’s even more awkward maybe you ought to stick with ‘girlfriend’s son’.”
Another reason:
I suffer from two chronic conditions that could result in my death. Now, it’s not LIKELY but it is possible. One time while Cohab was around I had a lengthy hospital stay when one of those conditions did nearly do me in. Apparently, when it looked like I was going to kick the bucket, my mom told Cohab he better start packing. He was living in my apartment that had my name on the lease that was being powered with utilities that were in my name. His stuff was in storage while we used my stuff. My mom made it clear to him that he had no standing and they were going to close my apartment down and hopefully he will have found another apartment of his own before they were finished. I realized if we had decided to keep his apartment instead of mine, and something happened to him, I could be legally evicted within a matter of days. Even if we had put me on the lease, the fact that I would have to basically turn over to his family anything that was his could put me in a very big bind. Even in our set-up, I would be in a big bind if something happened to him, because we had things like cable contracts, car payments, and stuff like that that I wouldn’t have been able to afford without his income, too. If we were married, it wouldn’t even be an issue.
And now I’m even older. I’m in my late 40’s, and everyone knows us geezers don’t like change! 
Plus, I no longer rent, I own. That would throw a really big monkey wrench into things. I would then have to either empty out and rent my house, sell it, or have my partner move in with me, basically rent-free (unless I started the nightmare of charging him rent!)
There’s so many things I would be willing to do if it came with the legal and emotional security that “piece of paper” brings. No, no one can guarantee a marriage will last, but at least the law is there to help you pick up the pieces. And a marriage is a lot harder to throw away than a girlfriend, so people tend to work harder at making them work.
But after both my long-term relationships, one with and one without cohabitation, I really don’t think I would or could do it again.
It’s not a matter of being “set in my ways” because I have no desire to NOT get married some day (although at my age I have accepted that I probably never will). I just don’t want to give up so much with no guarantees or legal or emotional or social support again.