I can't speak for what "most" of a population does, not having polling data, but, anecdotally: A friend of mine lived as a butch lesbian until middle age, then transitioned to live as a man. After transition, he partnered with a femme cis woman who has also identified as a lesbian all her life. They've kept all their lesbian friends, so they're still part of that community, and they call themselves a "queer couple." Each of them, for their own reasons, feels a loss in describing themselves with the word "straight" even though that's how they present to the world and arguably it's a result of their life choices. "Bi" would just be inaccurate. They can't use "lesbian" because it would reveal and undermine the gender of the trans man, preventing him from being mostly private about his gender history. "Queer" is a way of alluding to the fact that their life stories in this regard are complex, and also that they are sympathetic with the disruption of cis/heteronormativity. So I think that some straight trans people do use the word "queer" as a way of dropping vague hints about sexuality and/or gender without outing themselves in any specific way.