Your early 20s bring a lot to the table; I'm talking about college, finding a full-time job, moving out, etc. For some people, this time of life also brings on relationships that could, quite possibly, turn out to be "the one." These are all things that I've got going on in my life as a 22-year-old, and on a daily basis, I see my peers doing the same things...but in a drastically different order than I am. One of the major differences I'm seeing is the order in which people are moving to "I do's" before moving in together.
This is why I think you should ALWAYS move in together before taking the marriage leap: you don't really know a person until you've shared a space with them.
This is something I learned when I moved in with my boyfriend. Charlie and I have been dating since we were sophomores in high school, and we can both agree that we learned new things about one another when we moved in together as college seniors.
For example, I learned that Charlie is really particular about the way the kitchen is set up, and he likely learned how much of a slob I really am. I learned that he likes to leave his socks all over the floor, and he learned that I will leave books lying everywhere around me.
On top of all of these minor things, I learned that Charlie is not a morning person. I'm not a fan of them either, but I have never seen anymore more angry than Charlie when he's woken up for a morning class and hasn't had 2 cups of coffee. On the flip side, he got to see that I genuinely get irked if someone interrupts me when I'm sitting down to write my novel, or read a book. I understand now how my significant other operates on a daily basis, and how my schedule and my needs play off of, and effect, his way of living.
After living with him, I've gotten to see what it would be like to live with him for the rest of my life, without having to make a legal commitment beforehand. Luckily, in my case, I don't see any issues with the way my relationship functioned in a tiny apartment.
There were, of course, changes that were made, but they were good ones, like falling into the routine of making Charlie a morning coffee if I woke up before he did, or him having waited to watch a funny video with me if my classes ran later than usual. We learned to adapt to one another, and it strengthened us.
I can't imagine what it would've been like to move in with him and figure out he was a nightmare to be around, only to have become legally bound to him. I think that everybody, if given the opportunity, should live with their partner before ever deciding to spend the rest of their life with them.
Ultimately, I know that everybody's life path is different; while I've gotten to live with my significant other before any major life decisions, I do know people in my life who have chosen to wait to move in together after they have gotten married.
If this is what has worked out for you, then that's great, but I genuinely do feel as though moving in before moving to "I do" is the smartest approach available.