Welcome to the wonderful world of chess...it's really surprising, if not flat out unbelievable, that you haven't discovered OTB that chess attracts a higher-than-average percentage of wackos, weirdos and a**holes than the overall population. Of course this is magnified considerably online where people don't have to be rude or ignorant to anyone else's face and thereby risk losing being told to commit an anatomically impossible act upon themself, or maybe in extreme cases losing a few teeth...sorry, am I coming across as a tad bit cynical here?
Maturity Levels

At heart grown adults are children but they have learnt to control emotional outbursts. These outbursts are much more likely if you are playing a person you cannot see and do not know.
For my own part I have enough imagination to know there is a real person behind the moves, but some chess players have little imagination for anything but chess.
Also you must remember that chess is the 'be all and end all 'to many; all the rest of human exsistence is 'dust and ashes'.

I have encountered rudeness OTB in tournaments, but it is probably more common on the internet. This issue has been addressed in many posts before, the problem is that the people who agree with you have been behaving in a sportsmanlike fashion already and the others are unlikely to change.

Not just in chess, but people are ruder in general on the internet than they are in real life. I think most of it has to do with the anonymity of being online. Few of them would act that way face-to-face.

While I don't approve of some peoples rude behavior, there is nothing anyone can do about it and it is such a non-issue that I don't understand why people complain.

I've come to new terms with the phrase "maturity levels"....These people have nothing to do with being mature, they are flat out a$$holes.

I'm not brandishing. I'm being completely honest. I try and give everyone the benefit of the doubt, but the longer I'm on this site, the more I see it. The majority of people that play on here serisously have no integrity. They are inflametory, rude and just flat out obnoxious. I'm calling them like I see them. Calling someone an a$$hole is not being immature, it's making an honest observation over a long trial period. I coughed it up to maturity levels when I first posted this, but have been trying to come to grips with how the majority of people I play act. It goes far beyond not saying "gg". Its people cursing at you when they lose, calling you names when you get in better position, disconnecting and leaving comments on your profile about your mother. It has nothing to do with poor sportsmanship - these people get off on imflaming others. They are flat out a$$holes.
I'm sure this post has been addressed before, but I feel it may help us all to bring it back into the light. I play OTB chess all the time and have been playing for years. I don't believe I have ever (and I really mean ever) played someone who has been discourteous or even flat out rude to me. A large part of chess (for me anyway) is learning from the competition. If someone betters me in a match, there is almost instant respect built. And after each match, a "thank you", "good game" and handshake ensues...
Now, online chess has somewhat differing results from my experiences. It seems the overall response to a lost match is resentment. As a part of my routine when playing a match (mostly out of curiosity), I check the players profile. Most fill out their date of birth, not all but most. The sad truth to all this - the players showing the largest resentment to a lost match or losing position are adults! This is shocking to me. How a grown man (and its always been men) can justify disconnecting, or having a hissy fit over a lost game seriously confuses me. Or whats just as bad, when someone wants a rematch for "vengence" but cannot say "gg" to the prior match. Has the Internet inflated our egos that badly that this is what it has come to? Grown adults acting like children when they lose? I belive it is time we remind ourselves of the ettiquettes we learned as children and remember that chess is still just a game to enjoy.