+1 - threshold of proof is pretty low for you two. When you write the relationship history, just write in there maybe just a few lines about your period of friendship... "John and I first met in 2007 at a friend's barbecue, and hit it off, but our relationship never progressed beyond friendship, until 2011 when we started hanging out together more just the two of us. After a few weeks, we became an official couple, and after only 4 months together, John took me out for dinner and proposed. We were married three months later. While our period of dating may have been short, we have been friends for so long that it feels like we'd been a couple for significantly longer than just 7 months. This year we are celebrating our fourth wedding anniversary, and we could not be happier."
or you know, whatever.

Somethign like that but a bit more expanded would be absolutely fine.
