"Why do people consider *insert word historically used in a derogatory manner* offensive?"
Like...?
To be fair, it seems that those around you no longer feel that it's offensive and/or that those you know have "reclaimed" the word, thus giving you the (false) idea that it's inherently no longer hurtful, but you have to realize that this isn't the case for, I would argue, the majority of people. It's still a very offensive word to the majority of gay people I'd argue, and I'm not quite sure why you seem to have a problem with that.
