Thank you for your reply. You've raised a very good point. In this case though, it was a church document that classified them as illegitimate. So perhaps it was the reverse. I suppose it is possible that they did marry but it just wasn't recognized by the Catholic Church for any one of several different reasons. Perhaps they were of mixed religion, or married by the government in Tunisia (they did live there), and maybe the Maltese Catholic Church just didn't validate it. Whether they actually married or not, I'm not really sure, but they did have at least four children together over 10+ years. So I'm inclined to think that they weren't just "living in sin". I suppose it was more complicated than that.
Maybe I'll get to the bottom of it one day. At any rate, thank you for helping me look at it in a different way.