I'm not advocating for any legislation or prosecution at any point in this conversation. I'm asking for measurement of the relative value of rights over responsibilities within a society.

To somewhat answer your question, would think legislation was needed to address the reply. It's an option that's being offered, horrible as it maybe, that could be argued is being asked for. As you rightly suggest with your example, it's a really tricky thing to get right. In the end, personally speaking, I would lean on the social aspects of the exchange. The person being told they are fat / ugly asked for the view if the other, and they have learned that. The person saying those things could be nicer, or kinder, and should expect a fair judgement from any involved peers for the comments they made. And for the clarify, what I mean by judgement is that others have an opinion of them based on things they have said / do. If I overhead that exchange, I would think the person saying those things is a bit of a dick, and the person hearing them is better off not being around them.