"Women's fiction" has a variety of features associated with it. The more of these features a story contains, the more likely it is to be classified as as WF. But no one of them is necessarily sufficient unto itself. A certain confluence of material is needed. These features include but are not limited to:
* Written by a woman, a person with a female-shaped body, a person presenting as feminine, or someone believed to be any of those things regardless of their actual sex/gender. People argue a lot about the "isness" of womanhood, and that's one reason some writers (of all sex/genders) use pen names (of the same or another sex/gender, or to obscure theirs as with initials).
* Aimed at a female audience. This is the subset of material most likely to be named WF on the book jacket and promoted as such by the author and publisher.
* Written primarily or exclusively about women, girls, or other feminine characters. Almost any story with NO male characters will get filed as WF.
These top three features comprise the triumvirate of "by, for, and about" that make up the lion's share of women's fiction. Anything with two or three of these is almost certain to be described as WF.
Other features include:
* About topics widely popular among women, or believed to be so. Some of these are human nature, such as an interest in family life from the female perspective; others are stereotypical, such as makeup; a few are ideological, such as feminism or gender studies. It is this feature which most tends to drive away readers who do not identify as women, and indeed, actual women or female-bodied people who are just not interested in the stuff society thinks must go along with that body shape.
* Badly written. I mean really, horribly, painfully bad -- like crack fanfic bad. (I used to edit a women's spirituality magazine. I've seen it.) There is a natural phase in all identity literature where most of it comes from trait-having-people who write rather than writers who have trait. So they're not experienced at writing and not good at it. They just have things to say, so they say them. Some of them eventually become good, but the early works in most canons are mediocre or bad. Women's literature in general still has a reputation for this, for various reasons, among them that it's still harder for women to become professional writers compared to men.
* Some things which some women like go very much against the grain of how most writing works. Some of these things can be brilliant. Others are extremely difficult to do well and easy to do badly. And most people don't even know what it's supposed to look like when it's done well. This contributes to the above perception of women's writing as "bad" but is actually a separate facet.
* Written by an author known for women's fiction, genderfic, gender scholarship, etc. This can be a great way of finding books, but it's a nuisance when people assume that's all the person writes and then suddenly they do one in a totally different genre that doesn't fit with the rest. It's a common area of mistaken filing.
* Genderfic about womanhood or femininity. This can actually be written by authors of any sex/gender, and may support or subvert stereotypes of any given time/place/culture. Some of it is quintessential women's fiction and some is stuff that many women criticize and wish would go away. Some of it makes men and masculine people incredibly uncomfortable too. Lots of genderfic is subversive stuff. But some of it is about the radical women's agenda of ... having a life.
Now, there's nothing wrong with having a "women's fiction" section if people want to write and read things which are largely by, for, and about women on topics that many women find interesting. The problem is when this genre gets treated as less than others -- especially, less than the "men's fiction" genre which chiefly consists of crackfic about punching people in the face. There's nothing wrong with adventure fantasy either; everyone deserves beach reading of their own taste. But it's not good to say that women's fiction is bad because it is by, for, and about women nor that men's fiction is good because it is by, for, and about men. It's not good to shove things into the WF section just because they were written by a woman, or someone believed to be a woman. This does a disservice to readers who are, in fact, looking for that confluence of things commonly considered women's fiction who will be annoyed by something out-of-place due to the author's crotch shape. Sometimes women or female-shaped people write stuff that has nothing to do with any of that, and some write in a gender-neutral or even masculine tone. (You can have lots of fun with a gender guessing program -- my writing spans the whole range from masculine through mixed to feminine in tone, depending on the character and topic.) It's also not good to imply that topics not characteristic of women's fiction are things women and female-bodied people should not be interested in. Stories are for everyone.
As a basic rule, don't misgender people's books. It's problematic for some of the same reasons as misgendering a person, plus some others based on content relevance and reader expectations. People use genres to find batches of stuff that are similar enough so if you like one example of it, you'll probably like others too. If a writer/publisher doesn't classify their work as women's fiction, it is usually a bad idea to file it there. Jacket labels aren't always reliable, though, so if you see a book that is by, for, and about women then filing it as women's fiction may be apt. If the author says it is NOT women's fiction? Don't put it there. That's rude.
However, you are free to interpret women's fiction as widely as you want, provided it has strong core elements (by, for, and/or about). There is no reason you can't write about nonstandard relationships, alternative family structures, or women soldiers who solve their problems by punching people in the face. Womanhood is what women make of it. So too, a genre is what writers and readers collectively say it is.
Over to you ...