Before my wife and i got married, we had both spent a good chunk of our adult lives as either single or in serial monogamous relationships. We both also come from a generational demographic where we watched the parents of about half our peers (my family included, although as in so many things, we were slightly ahead of our time) go from beaver cleaver to "rhoda" in the space of about 8 years.
I believe that much of this had to do with members of our parents generation marrying too early, for the wrong reasons, and generally having ideas about marriage that made it difficult for them to exist inside it. My wife and i used to joke about the "institution" of marriage.. I mean, an institution is something people are always trying to escape from, insn't it?
I think our marriage is good, in part, because we did both have the time to develop separate identities beforehand, we went into it with realistic expectations and with both eyes open... And we had good ideas about each others space and how to try to reconcile living with each other and not encroaching on each other too much.
That's not to say we never fight.. God, no.. But i guess my point is, it would be harder i think if we had some external definition of "marriage" and what we both thought we needed to shoehorn ourselves into to comply with it.
Thats not saying thats what is going on with you guys, either. For all i know, the arrangement you're organically progressing towards right now may be exactly what's right for you guys. People have different ways of living and making things work, and that's cool.
Good luck!