I read the article. Cynthia Nixon did say that for her, homosexuality is a choice. And I agree with the article's author: she has a right to her opinion.
If I were bi like Cynthia Nixon, I would also say that I have the ability to choose between being in a relationship with a man or a woman, since I am attracted to either one. My attraction to a person would not be influenced by their gender. After having been married to a man for 15 years and now engaged to be married to a woman, Cynthia can hardly say she is hetero. Even if she "chooses" to be hetero, her history proves that she is not. Her gendered sexual preference is flexible. A hetero person's is not.
Likewise, this does not mean that Cynthia could be attracted to every woman on this planet, or every man. She will be attracted to some people more than others and in this, she also has no choice.