I believe that can be done without being offensive, rude, or overly judgemental about specific situations. Your comments about the arrangements being wrong are, IMHO, correct...your extension of that into the poster's personal motivations and impact to his children went well beyond that.
It's interesting that the people who are most concerned that their views will offend others are invariably those who support traditional moral values. When they don't just keep quiet and "mind their own business," they seem embarrassed by what they stand for . . . and they readily offer those who disagree some possible easy-ways-out, especially the "Now this may not apply to your own situation . . . " line. (That very line, in fact, from a poorly-worded research summary by David Popanoe, was a big hit among the pro-shacking crowd on this thread.)
Meanwhile, those who want to destroy the family and family values in this country are adamant in their insistence that theirs are the only enlightened views imaginable. They take offense at the most mild criticism of their behavior, even when they themselves have held out that same behavior as an example for others to follow.
I am convinced that what we need are more social conservatives who are out front about their values. If only one in a thousand prospective landlords expresses misgivings about renting to an unmarried couple, the result will range from outraged offense to lawsuit. But if many or most landlords would say, "I don't never allow people to shack up under my roof," some of this nonsense would end.