Because we were talking about couples with children. I don't see much moral or practical difference in child-rearing ability between an exclusive, non-married couple and a married couple. In the broader scope there are certainly practical differences between an exclusive, non-married couple and a married couple.
In a larger sense, though, I'm starting to question my purpose in making that statement at all. I'm not sure what sort of discussion I was trying to engender, or what the goals of such a discussion would be. I'm starting to think that I might not have been trying to discuss at all, but that I was just being argumentative and challenging because I was feeling defensive. That's not something I should do. I apologize.