I wouldn't agree that a comment is offensive if someone says they are offended by it. That means we all have to 'walk on eggshells' all the time. I think it's important to remember that some people take offence much more readily/easily than others. It could be that they feel they have a position to defend, or they think something important to them is being attacked; I don't really know. But look at the way extremists in Pakistan "took offence" at a girl with mental problems accidentally using some pages of the Koran to light a fire (I say accidentally because the news I've heard is that they could have been planted in her bag; she wouldn't have realised that they were pages of the Koran). The point with those people is that they were looking for opportunities to take offence in order to cause trouble.
Now, I'm not saying that there is anyone on gransnet like that, but that shows one end of the scale. At the other end might be someone who feels that what someone has said could be an attack on something important to them but they accept that the person speaking has a right to hold a different view, and to express it, however distasteful the different view might be to them personally.
What I'm saying is that I don't think "someone finding something offensive" is a good way of judging what is or isn't intrinsically offensive. I don't think saying that an idea or a belief is stupid is intrinsically offensive. I also think that, usually, it's fairly easy to tell whether something has been said with an intent to offend or said in the spirit of robust discussion in an ampitheatre (why did I think of that word?! What was the place in Greek cities that they went to discuss stuff?) of free speech.
I like my father's advice, which was that he felt it is more wrong to take offence when it isn't intended than to give offence. I think that is wise though not always easy to remember in the heat of the moment.