My Gf and I are "shacking up" as you call it. We were literally best friends, inseperable as a matter of fact, for a long time before becoming romantictly involved. We literally do everything together and have all the same interest in life(working out, running, cooking etc). I know for fact we have a stronger, more loving relationship than many married folks and many married people have told us that.
I know several couples who have never been married, produced several kids and have been together faithfully for 30+ years.
Good for you. I'm not such a prude that I'm going to condemn your committed relationship because you haven't married.
That's a different subject, though.