Ignorance.
Or well, no, it doesn't offend me, it just pisses me off.
I don't get offended, I either get upset on a personal level or I don't give a fuck. I think getting "offended" is just rubbish, it doesn't really mean nor do anything, it's just another way of acting like your feelings are more precious than everyone else's and everyone should treat you like you're made of glass and shut up about what they think.
Your feelings aren't that precious. They're fucking not. Calm your tits.
Instead of bundling up in a little ball, why don't you straighten up and stand up to the people who offended you? Or, if you don't want to spur any debate just walk away from the fucking situation.
It. Is. Not. Hard.