Why does this matter? Well, aside from the fact that claiming that we can choose to be queer (and, by extension, straight) fuels the barbaric practice of "conversion therapy," it can also have unthinkably terrifying consequences in other ways.
About a year and a half after I came out as a lesbian, I moved to Miami, where I fell in love for the first time, which would have been wonderful if not for the fact that the person I fell in love with was a man. How does a gay woman fall in love with a man?
If it were a choice, it shouldn't matter. It would not be a sin unless you choose to believe in such things, and it would not make the relationships any less valid unless you choose to view them as less valid.
Many would argue that discrimination against left-handers or homosexuals is indefensible regardless of the origins of those attributes, but human prejudices are complex, and the simple reality is that many people find it easier to accept a difference that is believed to be innate.
The best analogy for the question of choice and sexuality is not race or gender but religion. Among defenders of Cynthia Nixon's "choice" remarks are those who say that, just as Americans choose their religion, why can't we choose our sexual orientation?
It seems that the "people are born gay" argument is the only basis for others' acceptance of LGBTQ folk. As a result, we continue to regurgitate findings from scientific guilds to support our claims of "truth" regarding our worthiness and full humanity.
I know that my gender identity is most definitely not a choice. Had I been given a choice, I would have chosen a gender identity congruent with my body's sex. I have suffered enormous, unspeakable pain for decades from having my real gender identity in conflict with my birth sex.