On "I didn't mean it to be offensive, therefore it wasn't offensive"

If you say something sexist or racist or heterosexist or whatever unintentionally--if you honestly had no idea that there was anything wrong with what you said--then I agree that it's not a reason for people to get mad at you. It's an acceptable reason for people to avoid you, if they don't feel like trying to educate you, but it's not reasonable to get angry if you truly had no idea. Obliviousness is not necessarily your fault.

If, however, someone says, "Hey, dude, that wasn't cool, and here's why," then appropriate responses are "I disagree, and here's why," or "oh, I'm sorry!" If someone calls you out and your reaction is, "You're being oversensitive and not understanding my intentions," then you are being a jerk. Ignorance is only an excuse when you're still ignorant.

*g*

Hulu commercial: If you're happy and you know it, clap your hands!
Mike: *cheerfully begins to clap*
Hulu commercial: *thunderclap, ominous music*
Mike (sadly): Oh.