Respectfully disagree here. In my experience, Q is never a stand-in for any sexual orientation, but only for sexual orientations that are different from straight. Heterosexuality, which is definitely a sexual orientation, does not fall under the queer umbrella.

A better way of putting it is that queer is an umbrella term for all members of gender and sexual minorities.

Let's try not to fall into the right-wing talking point that straight people don't have a sexual orientation.

Also, queer is as much about gender identity as it is about sexual orientation. Transgender people certainly fall under the umbrella of queer, irrespective of their sexual orientation. Trans people can be gay, straight, bisexual, asexual, etc. But they count as queer because they are trans.