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.
Within the context of discrimination, one of the most used "weapons" of those against equality is the notion that people "choose" to have a homosexual orientation. It is important to challenge this myth, because it forms a key part of opposition to equal rights for gay and lesbian people.
We should be thanking Cynthia Nixon for giving us an opportunity to talk about sexual orientation for what is really is, a complex human trait that is not fully understood, and not a simple gay/straight binary but a spectrum of behavior and identities.
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?