No one likes to be wrong. No one likes to be told that something they do or so with no ill-intent, is in fact, incredibly offensive.
We're living in a time where people are becoming increasingly aware of the ways in which different cultures interact, and the way in which history has coloured those interaction. We're realizing that callous disregard for the lived experiences of other people is no longer acceptable. But when you've grown up in a world that tells you that something is okay, it takes time and commitment to adjust your worldview.
That's where "-isms" come in.
Bigotry and intolerance are big, bad, intangible monsters, and no one person can fight them alone. But what we can do is call out the people we love on their bigotry when it rears its ugly head.
I am personally of the belief that everyone is a little bit racist/sexist/classist etc. The world is set up in such a way as to reinforce those things as the prevailing accepted values, and anyone who reaches adulthood without internalizing at least a few of them is, in fact, a saint on earth. The issue isn't about just about not being intolerant, it's about actively analyzing our own behaviour to recognize when our actions are stemming from bigotry, and taking steps to combat them.
Impact trumps Intent. Every time. It's a concept that's surprisingly hard for people to understand. Basically, what it means is that the fact that you didn't mean to offend someone, doesn't change the fact that you offended someone. And when it comes to matters of race, class and gender, it's especially important to examine individual privilege and acknowledge that sometimes hurt is unintentional, but it's no less hurtful.
That's why I think it's so important to call out the people we love on their "-isms." For me, if I call a friend out, it's because I think they can do better. It's because I know they're a better person than one who relies on racial stereotypes or disregards the working poor. It means I respect them enough to give them the benefit of the doubt. It means I think they're good enough people to understand that though their offense was not malicious, it was still hurtful, and that they will adjust their behaviour accordingly.
If I call you on your "-isms" it's because I think you're good enough to do better.
And yes, it sucks sometimes to be the "oversensitive feminist friend", but privilege is real, and it deeply affects the way we are allowed to move through the world. Decent people will acknowledge their unearned advantages and try to call out inequalities when they see them. Bigots will relish in their power. I'm not really interested in being friends with the latter.
So, do you think you're a decent person? Then examine your privilege, encourage your friends to examine theirs, and encourage them to become people who are actively fighting against the many varying forms of oppression that exist in our society.
Refuse to tolerate bigotry in any form, big or small.