I think that part of the argument is that sexism isn't perceived as being as 'wrong' or serious an issue as racism. If people went on a thread here & made a racist remark, there would be a very strong response. Everyday, there are sexist remarks made which people don't even notice.
I live in the US, and people have commented to me that the UK had less of a feminist movement than the US - I have no idea how true that is, or how to measure that. In many ways, small town US is far more 'Stepford' than the UK, BUT I just don't hear comments about 'you throw like a girl' etc. It's just not acceptable to make those kind of off-hand comments.
So, it does seem fair enough to say that sexist comments should be challenged/removed etc. And there's a fine line between genuinely having a different pov, or asking a question to find out more, rather than trying to throw a spanner in the works & upset people. On the screen, it can seem impossible to tell the difference.
But it should go without saying, that on ANY sensitive issue, you wouldn't go charging in with opposing views that could upset people.
Style & Beauty is often referenced on here, as it's a more 'fun' topic, but even there, if someone was asking for style advice & said they felt bad about their body because they'd just had a baby, it would be seen as offensive to say 'well you look fat on your profile page, so just wear something baggy' etc.
Basically, people should just learn a few manners. It seems a bit sad that we even have to say that when discussing topic A, it's rude to charge in and start arguing that topic A shouldn't even exist, you're all wrong etc, whatever the topic is.