I read this a lot on reddit. Someone will preface their comment with "I'm going to hell for this" or "Apologies in advance" as if that's supposed to excuse it. I get the feeling it's because these people don't see themselves as ____ist, so when they do something ____ist it's "just a joke" or "I'm not serious!"
I think for us to get rid of this culture, we need to make these people realize they're actually sexist. They're not a nice guy making a sexist joke; they're a sexist guy making a sexist joke, and this is how everyone sees them.