Do you have a quote?

Don't have the books with me at the moment, but it's at the beginning of OotP when he talks about Snape mocking him for doing the cleaning and Sirius obviously feels that cleaning is a bit degrading. There are two ways to take this: it's house elf work or women's work. The only people in the books we see cleaning are house elves and Molly Weasley.

I would never dream to assume she needed to defy all of them to make a point

I think you misunderstood what I'm trying to say here. Hermione--inadvertently or otherwise--presents herself as better than Parvati and Lavender because she doesn't usually giggle and gossip. As a strong heroine, she can't be seen to indulge in giggling about boys because that's what teenage girls do and teenage girls are disparaged--in our culture and, I think, in hers. It's exactly the fact that there aren't prominent heroines seen to be flighty and gossipy that shows the sexism. In order for Hermione to be taken seriously she needs to separate from teenage girls who behave as typical teenage girls do. There's absolutely nothing wrong with Parvati and Lavender's behavior, but it is disparaged by both Ron and Harry and it is feigned in a sort of "I'm above this and am only using this as a manipulative tactic" way by Hermione.

I think in writing characters, an author can certainly take these minor differences into account.

Sure, sure. Again, I don't have the books with me so I can't look over the scene again to remind myself of exactly what bothered me. I think it's that I got the overall sense that Cho was a great girl--smart, pretty, strong, and loyal--and yet she is downplayed and overshadowed by her emotions. At the end of OotP Ginny talks about Cho throwing her broom down in frustration when she loses the match and there's this sense that she's being over emotional when she's really just displaying typical teenage behavior. But Ginny, in contrast, is, like Hermione, less emotional--less "girly"--and this is why she is taken more seriously.

but I still do not think it is a major issue for the Wizarding World, at least not in the UK, maybe it other areas of the world.

The way I see it is that sexism in the Harry Potter setting is about the same as it was in the modern Western world in that time period. It could be a lot worse, but it's not non-existent.