I think this is where we disagree, I believe a post on a forum making a bold statement should (quite literally) be spelled out, nothing should be left to be read between the lines, it isn't like having a face to face discussion where body language allows us to fill the blanks, read literally, the post states that if somebody is fat then they are being silly if they do not accept that people will call them fat.
You're probably right, the point was probably that if you're fat and a loved one says that then you can't be surprised by that as it's a statement of fact (albeit one that can be worded in a nicer way). However that is not what the poster wrote, so as a reader what else am I to do except assume they meant that if you're fat you can expect to be called fat by everyone and anyone and you're silly if you believe it won't happen.
My point was that while that may be the reality, it isn't right for that to be the case and the poster should not express that it is what will (and from the tone of the post I assume they believe should) happen.
The telling off (or opposing argument as I prefer to call it seeing as that is what a discussion normally consists of...) was deserved if only because the poster was unclear on a sensitive point.