I agree that being offended about something is not a matter of choice, it is primarily an emotional reaction which might then be rationalised intellectually. My point is that there are some groups who seem to believe that they have the right not to be offended and that they should be legally protected against such offence. The most obvious (to my mind) example of this is blasphemy legislation.
Being offended is not pleasant, hearing something which one holds dear being denigrated is deeply unpleasant, but none of us should have a legal right to protection from that. In my experience being offended has led me to having a more open and accepting attitude to the views of others because it has made me wonder why others do not share my world view and to read and listen to other views.
Protection from offence closes down debate. I know that some offensive behaviour is no more than puerile foot stamping or 'look at me' posturing but that too we just have to deal with.