Just because a demographic is privileged along one axis doesn't mean it is an unmarked identity, it simply has accompanying cultural constructs built into it that we're accustomed to viewing/treating as a "neutral" default. But there is no identity that is a "neutral" blank slate!
To start simply: M/F stories are not neutral, they are usually built on a scaffolding of gender roles and tropes that almost always center romantic monogamy and allosexism. To write an m/f story, you must choose whether you will engage with /subvert / challenge some (or all) of that. Whether that's an active choice or a passive/unthinking one, it's still a choice.
Likewise, male characters are not neutral, they are usually constructed around ideas of masculinity (sometimes toxic) and gender hierarchy, making them more dominant in our shared cultural consciousness. To write a story centering men, you again must choose whether you will engage with that baggage or unquestioningly include the default assumptions that come along with contemporary gender constructs.
See also: other identity axes. White characters are not neutral blank slates! Able-bodied characters are not neutral blank slates! Slim/conventionally-attractive characters are not neutral blank slates! Etc etc.
And things get even more complex when multiple identities intersect. Writing a romantic story between two white men doesn't "erase" any baggage from gender or race - it may challenge heterosexism, but the author must still choose whether that story replicates or engages with the other identity constructs involved (whiteness, masculinity, monogamy, allosexism, alloromanticism, etc).
There is no identity you can write about that is a "neutral" blank slate, even if you set your story in a different world/era without our contemporary hierarchies of identity, because you can't erase the cultural constructs and assumptions that you - and your readers - have inherited from our parent cultures.
This does NOT mean that you can't write stories about characters devoid of contemporary cultural constructs, just that doing so means making a ton of active choices, not unquestioningly replicating the hierarchy we've been steeped in all our lives and calling it "normal"/"neutral."
