I think it's that are just using a term that was always used. Feminists introduced gender to talk about gendered social expectations, identities, and so forth to show how there was no one-to-one pointer reader relationship between physical sex and the habits, identities, etc. we exhibited. Gayle Rubin's seminal essay, "The Traffic in Women: Notes on the 'Political Economy' of Sex," argued that the sex/gender systems was a "set of arrangements by which a society transforms biological sexuality into products of human activity, and in which these transformed sexual needs are satisfied".
E.g., there was a war in our department years ago when some people wanted to stop calling a course Sex Roles and, instead, talk about Gender Roles. The change signaled a change in theoretical perspective, from the structural functionalism once dominant in sociology to a feminist critique of SF.
In turn, it was Judith Butler who, in _Gender Trouble_ criticized the social constructionists reproduction of a foundational category in sexuality that feminists, elaborating on Rubin's critique, had rearticulated unwittingly - or so Butler argued.
Rubin, meanwhile, went off on her own to write the famous argument in "Thinking Sex" (presented at the infamous Barnard conference) against the possibility of any feminist theorizing about sexuality. They had too much invested in an essentialist understanding of sexuality, Rubin would later argue. Rubin was motivated to write this essay, born out of her experience in the S&M scene in Ann Arbor, where feminists wouldn't support queer struggles against the raids, arrests, etc. because they saw S&M, along with pornography, as the sine qua non of women's oppression.
Rubin's essay was critical to what became Queer theory because she argued that feminism simply didn't have the theoretical balls to concede that sexuality exceeded the binary of straight/gay and, thus, it was the task of what became Queer theory to create a theory and practice specific to sexuality. So, the point at which fluid sexual identities became a clarion call to queer "radicalism" was circa 1982.
There was a great set of essays in a journal, name of which escapes me at the moment, but it was an interview between Butler and Rubin in their theoretical differences become really clear. Janet Halley writes about it in _Split Decisions: How and why to take a break from feminism_ in which she is siding with Rubin ultimately, arguing that feminism -- and political and politicizing theory can't "see around it's own corners" so it's important to take a break from it and advance another politics for awhile instead. Her metaphor is that she "becomes a white gay man" so she can step back and assess feminist practice, specifically the effects of MacKinnon's power feminism, on gay men.
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