I'd say I agree with that.So, I believe you are saying that flashing should be considered an illegal offense only when it can be demonstrated beyond a reasonable doubt that the defendant was acting with the intent to elicit a reaction, and
Are you also saying it shouldn't matter what kind of reaction (fear, anger, disgust, excitement, admiration, amusement, arousal) the flasher hoped to elicit?
The problem you end up with is simple, and I'm sure it's the reason you included "demonstrated beyond a reasonable doubt". You have to differentiate between a case of a person being nude and ending up in a compromising situation (sitting on a train reading a paper with your wang out, suddenly lowering that paper and another passenger interpreting this as flashing) and a person who is actively out trying to get a reaction (exactly the same example as the one above only this time there's intent). It's damn near impossible to judge unless there is a clear cut reaction from the accused flasher.
So you then have to ask yourself, do they allow nudity and this rather nasty grey area in the law? Or, do they ban it and simply state any public nudity (other than in assigned areas) is considered indecent exposure?