This is the year of reading women, people, remember? We're all reading female writers and helping address the literary gender imbalance which is highlighted annually and disturbingly by VIDA. So everything's good, right? We're slowly rebalancing the world, book by book, as we tackle our teetering piles of Mantels and Atwoods and Cattons.

Sadly, no. The excellent Joanne Harris (have you read her latest, The Gospel of Loki? You should, it's a cracker) has explained in detail why it isn't in a blistering new blog post sparked by what she calls "another lazy assumption" from a reader who said the novel is "capitalising on the fandom of Tom Hiddleston".

Apart from the fact that Harris first wrote about Loki way before the Marvel films starring Hiddleston came out, she believes the comment is the tip of an iceberg. "A great big iceberg of sexism within the whole book industry, which stealthily perpetuates the belief that no woman writer can ever really be successful without having somehow copied from, used or otherwise capitalised upon the popularity of a man."

"Imagine someone accusing Salman Rushdie of 'capitalising' on the folk tales of the Middle East. Imagine someone accusing Neil Gaiman of 'capitalising' on the popularity of: Norse myths; Doctor Who; Claire Danes; milk. Imagine someone accusing Lee Child of 'capitalising' on the popularity of Tom Cruise," she writes, before detailing the various assumptions which have been made about her throughout her career.

The situation isn't helped by the likes of TLS editor Peter Stothard or VS Naipaul, Harris says; by the use of terms such as "chick lit", or by male academics' dismissal of female authors. All true, but what really caught my attention was her claim that "'Women's fiction' is still considered a sub-category. (Amazon; Goodreads; Wikipedia; take note)".

I knew it was - or had been - on Wikipedia. There was a controversy about that last year. But Amazon? Really? I checked it out; she's right. There's a category for "Women writers and fiction" on the site, and within that for "Women's literary fiction" – hi Rachel Joyce, Charlotte Mendelson, Maeve Binchy, Kate Morton and Virginia Woolf – and "Women's popular fiction". I'm bewildered by how titles make it into these categories. The mix of books is so broad as to be meaningless, united only by the authors' gender. But the fact remains the categories are there, and there are no equivalent "Men's writers and fiction", "Men's literary fiction", and "Men's popular fiction" sections. They are just "fiction", I guess.

Goodreads, meanwhile, has a hugely diverse list of genres to pick from (wizards or Spider-Man fiction, anyone?). "Womens" and "Women's fiction" both feature, but no equivalent men's labeling.

I asked Amazon to explain their reasoning; I didn't hear back. I asked Harris why she thinks it is an issue and this is what she told me: "It's an issue because effectively the gendering of books excludes certain readers from an area they don't need to be excluded from … Women aren't a sub-category … When you say literature it seems to me there is a definite implication it is written by a man. That is absurd and ludicrous but it is everywhere. It is a general and very broad strand of prejudice."

If there is women's literature, points out Harris, why not men's literature? "Why does fiction need to be gendered? ... How good does a woman writer have to be before she is referred to as a writer?" (Hilary Mantel has got there, she says, and so has Margaret Atwood.)

Perhaps there's something in the air, because Harris isn't the only author enraged by this. Randy Susan Meyers blogged earlier this week for the Huffington Post about how "if you want to publish on Amazon, you must pick a category from a wide-ranging list of possibilities that includes 10 subgenres of women's fiction and zero that are labeled 'men's fiction'".

It's clearly a marketing decision, I thought, so I asked Cathy Rentzenbrink, the associate editor of the Bookseller, if she could explain. "As a person, a feminist and a reader, I completely understand and feel the frustration, but practically, I also know there are vast amounts of real people who want guidance towards the sort of book they will enjoy, and that is what publishers and retailers are trying to provide," she told me. "Even the dreaded 'chick lit' term is useful in that the reader who wants that type of book knows what they are getting. It's a bit similar to the genre debate. I always enjoy lofty cries of 'There should be no genre, there should only be books', but those of us who understand the coalface of bookselling know that a large building with no categorisation other than 'Books A-Z' would be very difficult to navigate."

That's true, but do we really want to give out what Meyers says is a clear message: "Men are the norm. Women are a sub-category". Meyers points out that "we don't need firemen and firewomen – they're all fire fighters. And all those writers we love? We don't need to call them writer-men and writer-women. We can call them writers. And we can call the novels they write, just that. Novels."

Harris, meanwhile, is clear that this is a discussion "which needs to be had, and much more widely than it is at the moment". As she puts it on her blog: "Don't let it go," or else "the trickle-down effect of sexism in the book business will continue to apply, on Goodreads, on Twitter, in bookshops, on blogs."

Because "'women's fiction' is not a genre".