To be fair, it's not an insult as much as an assessment. And considering your post he was replying to, his assessment isn't very far off.
Using a "slippery slope" argument in a thread centered around the NFL banning fans for racist heckling shows a complete lack of reason.
This isn't a first amendment issue and it's pretty common knowledge what kind of language will not be tolerated.
It's fairly easy to avoid saying racist things if you're not a racist,so you nor anyone else should have anything to fear with regard to a "slippery slope".
Often times when arguing a larger ideological point, people can lose sight of common sense. And I think that's what you've done here.