I'm not going to debate on which provides durability that is immaterial to the question at hand which is "Is living together totally acceptable?" Today's society for the most part accepts it, marriage is cheap divorce expensive, and marriage is the biggest predictor of divorce now you do the math on why people don't get married. I think you answered your own question when you said that the couples were perplexed as to why you were even asking if they were married. If you just wanted to brow beat people with your own opinion then by all means continue to do so if you want an answer to your question I have given you mine.
marriage is the biggest predictor of divorce
UHHHH. This doesnt make sense....one cannot have divorce without marriage.
On the other hand...one can have "Moving out" when one is playing house...and there are a lot more move-outs than divorces.