The Sex and the City actress recently made a speech to a gay audience, where she wanted to include the line, "I've been straight and I've been gay, and gay is better".
She was asked to change or remove the remark by the event's organisers, but she refused, standing by her claim that she made the choice to be a lesbian.
"I understand that for many people it's not, but for me it's a choice, and you don't get to define my gayness for me," she told the New York Times. "A certain section of our community is very concerned that it not be seen as a choice, because if it's a choice, then we could opt out.
"Why can't it be a choice? Why is that any less legitimate?"
Her comments have been met with criticism by gay bloggers including John Aravosis, who said: "What she means is that she's bisexual, and doesn't quite get that most people aren't able to have sexual romantic relationships with both men and women because they're just not into both genders.
"She is into both genders. And that's fine. But she needs to learn how to choose her words better."
The actress has been in a relationship with education activist Christine Marinoni since 2004. She was previously with English professor Danny Mozes from 1988 to 2003, and they have two children together.
Cynthia Nixon is currently starring in the Pulitzer Prize-winning play Wit, in which she plays a celebrated poetry professor battling terminal cancer at the Samuel J. Friedman Theatre in New York.
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