Well, that’s not how this author sees it. 
From my perspective, I’m still 100% free to tell the story I want to tell, even when I share agency with the reader–indeed, I’m writing IF because there isn’t a single fixed story I want to tell, but a narrative landscape I want to explore along with the reader. That landscape includes some characters whose sex, gender, and orientation are a core part of who they are, and some for whom those things can vary without compromising my vision for the characters.
In general, I doubt that too many authors’ freedom has been compromised on this issue – there’s a host of games out there, both CoG and HG, with fixed-gender or fixed-orientation ROs, and (for authors who might be weathervaning on which way is most popular) ample evidence in these forum discussions of an audience for either approach. The recent kerfuffle over Golden Rose wasn’t about Ana being forced to go along with genre norms of playersexuality; if anything, the strongest external pressure she faced was from the share of the audience that valued fixed-orientation ROs.
I recognize that writing playersexual characters puts a constraint on some players’ freedom: freedom from choices that break their immersion and go against their aesthetic preferences for the fiction they read. Of course writing characters this way expands other players’ freedom. An author should be free to run with their own preferences, while being aware that nothing they do will satisfy everyone.