This is very interesting and I think the kind of slash fiction that used to be the case (eg often slightly older women writing two male characters together in fan fiction) probably does have an event of imagining a relationship at least partly free from gender roles.
But the difference with the current mode of female teenagers doing it is that they are largely completely steeped in a pervasive youth culture of designating everyone as a “bottom” or “top” and each character as taking on either dominant or submissive roles that do - even if they are not fully aware of it - mimic the “traditional” gender roles, only this time viewed through a lens of quasi-BDSM and some rather mistaken yet completely codified ideas about how all same sex relationships (in fact how all relationships) have to necessarily operate along some kind of dom/sub/bottom/top axis.
This is a relatively new thing but something that really is pervasive in young people’s ideas of sexual culture, even if they haven’t had sex and have no real romantic experience with the opposite or same sex! Just as the gender culture has proliferated all these gender labels, so sexual behaviour has been proliferated too and gained all these labels as well - so you have twelve or thirteen year old girls declaring themselves a “Demi-boy trans masc sub bottom” or whatever, and imagining their favourite boy characters in similar terms.
And all these “roles” are imagined as separate to gender/sex, but inevitably they carry along with them a lot of buried assumptions that are connected to gender stereotypes, so the roles of “top” and “bottom” get imagined as masculinised or feminised even in imaginary same sex pairings - so these are no longer imagined as places where ideals of equal relationships can be explored, but rather the reverse - as place where some of those “roles” actually get acted out in what ends up being quite a reductive and reactionary way. For example, girls imagining two boy characters in a same sex relationship - but one is the “top” and one the “bottom”, and the bottom enjoys receiving sex and being feminised/choked/submissive and being dominated by the “top” - and right there, suddenly there are some not very equal roles back in play, except it’s all supposed to be terribly “queer” and progressive because the roles are separate from sex/gender (only — they aren’t really; it‘s just really like a revival of the old stereotype of “who’s the man and who’s the woman?” that many people used to have about same sex relationships).
Sorry, long post! But I think it’s crucial in understanding these new forms of imagined pairings that girls are “identifying” with not to think of them as necessarily liberating, in the way that writing same sex slash perhaps was for adult women - but seeing them instead as part of this new culture of having to define and label every aspect of sexuality and gender as something “innate”.
You can’t just be a young person exploring sexual fantasies any more -- they always have to be terribly rigidly bound by all these definitions of “I’m an aro-ace demi switch pan femme” or whatever; and woe betide you don’t say the “right” things for the “right” role, or someone will start arguing with you that you aren’t doing things “properly”.
In vain you will try to explain to a teenager that most people, including in gay male relationships, do not see themselves as innately a “bottom/top/dom/sub/switch” or whatever, and instead enjoy a variety of sexual interests and roles in bed without this being an essential part of their gender, identity or personality. They do not like this idea 🤣 Possibly because if you get rid of these ”roles” as somehow innate, it also threatens all the whole ideology of gender identity along with it.