Hmm, this is an interesting, yet difficult question to answer!
We knew early on that we saw and wanted each other as life partners, in the sense we knew we were committed to each other and to sharing a life together.
We had always had open, and honest, communication with each other about everything, including our expectations, our feelings, our desires, had talked about all those issues that couples ought to talk about if they are considering a future together, and so on. Not with a purposeful approach as in “we need to talk about this to decide if we are right together” but in a way where communication just flowed naturally and we realized at end of day we had really shared everything we could share with each other (but of course, this is a lifelong process, just because we talked about it once, does not mean we shall never talk again, ha!).
Also, when we met we were both rather “settled” into who we were as people. We met when we were both a little older and had a good foundation of life and relationship experience by that point, were reasonably self-aware, and had a good sense of what we wanted in a lifetime partnership. I think we were able to assess our own relationship in a healthy, balanced way at that point, and both knew we had found in each other something pretty special.
Marriage was not something that I personally felt was important to me, but I did want that committed partnership. This is just mostly from perhaps how I was raised, my own perceptions at the time of marriage, and so on. In any event, ultimately, I was quite happy not ever getting married so it was not something that really tended to cross my mind much.
However, my husband did ask me, rather spontaneously in some interesting circumstances. It was a surprise in the sense that the proposal was a surprise (it was a ringless one, as well), but not a surprise in the sense that he wanted to spend his life with me, and he knew I wanted to spend my life with him too. And I said yes, because to me there was no question in that moment that was what I wanted, too.
So, I think it was a bit of a combination of both. Being together had always just felt “right” and it felt completely natural to make a lifetime commitment, whether legally or not. That, combined with lots of communication, meant we were both on the same page and committed to each other and our relationship. The actually decision to be legally married came after a lot of communication, but it itself was more of a leap of faith on both our parts as it just felt right (and still feels right several years later!) We had a short engagement, so once we had decided, yes, we were going to do it, we wanted to do it as soon as we could!
As cheesy as it sounds, it feels like we always were just meant to be together, in whatever form that was. Whether it was with a legal marriage or not was unimportant to me, but when he asked me it felt like there was nothing else I wanted more.