The lesson we learned was how to handle being offended: to challenge the offender directly, find a resolution, without involving an internet mob.
OP did this the correct way, and you reacted with hostility instead.
"Retarded" is a word that the bullied (and their loved ones) have heard all throughout their lives used in a pejorative manner. It is also rarely used IRL in a non-pejorative context, even though the definition exists. For someone to associate the word with its pejorative and insulting context is not at all unreasonable.
"The only winning move is not to play"
This is why we don't use the words "nigger", "kike", and "faggot" as insults anymore, why calling somebody a "pussy" or "woman" for not being aggressive or assertive enough is generally frowned upon, and, yes, why casually dropping the word "retarded" like it ain't no thang will get people for whom the word actually means something politely asking you not to use it anymore.
Communication is a two-way street, and it's up to the listener to determine how they interpret the words coming out of your mouth. I'm amused and irritated by how many people get frustrated when they're told that their choice of language bothers somebody. It's like, hey, here's an opportunity to refine how to interface with the world! And a bunch of people see that opportunity and go FUCK NO, I'M TIRED OF HAVING TO DO THINGS, SOMEBODY ELSE JUST STOP BEING OFFENDED OKAY
If you think the correct answer is to try to get everyone everywhere to not use whatever subset of words you subjectively find offensive versus just IGNORING THINGS THAT OFFEND YOU, I'm not really sure what to tell you, other than perhaps:
Good luck with that, you retarded faggot.[1]
Many other people are not aware of that offence, and also would not like to cause that offence, and once they know that many people find the word so offensive are happy to consider their use of language and find another word.
There are lots of great words to use instead of retard and retarded.
And asking someone to consider their language isn't asking for an Internet pile-on nor asking for anyone to lose their jobs.
Seriously. Could you stop that please?
What we should be talking about is where exactly the line is drawn, not whether complainers inherently suck.
They are subjected to verbal abuse, bullying, sexual abuse, and physical violence, sometimes from the people who are supposed to be providing care for them.[1abcd]
People with learning disabilities suffer poor health outcomes, and those are not all explained by their LD.
People with LD are deprived of their liberty, detained in hospital against their will, without trial. People get caught up in bizarre Kafkesque nightmares.[2] That man was held for a year, with all of his rights ignored.
People with LD are illegally discriminated against in the workplace assuming they can even get a job.
Given all this the pain caused by people carelessly using the word 'retard' trumps your supposed offence at my enabling 'complainers' so I say, politely as I can, FUCK YOU.
See, that's the point. I don't care if you know you are being offensive and want to cause offence. Go ahead. My post was clearly aimed at people who do not know how offensive some words are.
[1a] (http://www.guardian.co.uk/society/2006/jul/05/longtermcare.g...)
[1b] (http://www.communitycare.co.uk/articles/31/07/2006/55162/cor...)
[1c] (http://www.mencap.org.uk/outofsight?gclid=CIziubGwkrYCFeXLtA...)
[1d] (http://www.dh.gov.uk/health/2012/12/final-winterbourne/)
Have we learned nothing in the last 24 hours?!