Ok it's pretty simple.
There should be rules of what is acceptable or not in guild chat, and when interacting with other guildies. If there aren't any, the guild should give you a little slack (I don't know your situation, i'm talking about in general).
And in that case, if you offend someone (and they show offense), you cut it out right away. You should get a warning if you offend someone, unless the rules are more specific. Lay off the insults at least until you get to know the comunity within the guild. Keep the remarks to your friends, because it isn't really that funny when it comes from a stranger.
And I am VERY sure that when you get to know the guild and become a long time member to know the majority of the older members, you can probably insult away and they will laugh and/or retort in kind.
If that's too much to ask from you, then you will probably need to find a guild who doesn't put any rules against insulting/offending people in their chat.
Of course, you can't really PREVENT yourself from offending someone. For you to know they are offended, they need to show it. If guilds punish you for that right away, then that's rather rediculous.
Personally, I would put guild chat rules in. It's just how I would run my guild. Hell, I've been in a guild where profanity is against the rules in guild chat. I don't remember his justification though, I think it was due to children in the guild or something (swears were running freely in teamspeak during raids). It suited me really, I don't swear at all, (ok not literally never but you know what I mean) and it worked perfectly. Sure it wasn't a big guild, but the chat regulations were followed seamlessly and without conflict. A couple of slurs came out (usually because the GM wasn't online lol) from other members but if they got caught a swift warning came in place.
Also, to be honest you shouldn't really judge guild policies if you don't know the GM. It's their guild, their rules. Run your own guild if it's a problem.
EDIT: One more thing: You might be taking it too literally. It's to stop the people who are actively trying to insult people (you should know better than to throw insults at people you don't know).
On a side note, making fun of druids offended people? That is really stupid.