The opposite of LGBT is not straight. The opposite of LGBT is not heterosexual. Sexual orientation is not the topic raised when we talk about people being LGBT.
I cannot count the number of times I have heard or read people use “LGBT” in one breath and “sexual orientation” in the next, completely conflating them. I cannot count the number of times I have heard or read phrases like, “Compared to their heterosexual peers, LGBT youth…” and “LGBT people are more likely than heterosexuals to…” Same goes, over course, for all the aconymous variations–LGBTQQIA, etc.
I appreciate that people are trying to be inclusive. I’m glad that LGBT is now the politically correct term. But unthinking inclusion is meaningless. What is the point of tacking on the “T” if you’re still going to talk only about gay and lesbian (and maybe, occasionally, bi) people? LGBT refers to marginalized sexual orientations and gender identities. The opposite of LGBT is straight and cisgender. I don’t buy for a second that this is purely a question of semantics. It is no coincidence that trans people are both linguistically elided and extremely marginalized, often even within LGBT organizations.
The T stands for transgender/transsexual. Transgender is not a sexual orientation. LGBT is not a synonym for gay.