I think a lot of people are easily offended because they are not comfortable in their own skin. This is to say that many people are quite insecure and any quip or comment that is not obviously flattering, may lead them into a tail-spin.
Insecurity arises from the lack of confidence and from the fear one may not be accepted. The internet is a place where Dr. Jeckels can turn into Mr. Hydes, because of complete anonymity.
Let the user beware. Not everyone will agree with you and not everyone will like you...get used to it - it's just a way of life. As Thomas Jefferson noted, "One-third of the people will like you no matter what; one-third will hate you no matter what; and one-third won't care either way." (loose translation).
Prevention is a tough road to hoe, because one cannot account for all the insecurities the other party has, nor does one know their background. To be offended is a choice and the only real prevention is to be confident with who you are and what you stand for.
It will also help to know that other people have different opinions that may conflict with yours. Be gracious and allow them to maintain their point of view and dignity. Those who prove themselves to be "drive-by" morons, should be dismissed. Allow their caustic remarks to roll off you like water off a duck's back.