While discussing hypothetical situations or speculating towards the reasoning behind some rule or behavior, my fiancée will sometimes put forward an idea with an obvious flaw in it. I will point out this flaw with the expectation of her correcting it to improve the idea, but instead she will respond with "Well, what do you think, then?" Often I don't have a hypothesis of my own, and will admit it, but she seems to think that it's unacceptable of me to criticize an argument if I don't have a better one to replace it. In general, am I being logically rude by pointing out that an idea is illogical if I don't have a replacement to offer, or is it acceptable to say, "I don't know, but I know THAT's wrong"?