Unless one person has some level of authority over the other, I don't see why this should be considered inappropriate. Not taking no for an answer, or any kind of retribution for being declined, are harassment and shouldn't be tolerated -- but just asking? I think that's something any man or woman should be able to ask any man or woman.
For avoidance of doubt: Asking someone out when you know very little about them isn't smooth. It won't impress many people. But I don't see it as morally repugnant.