The Sex and the City star enjoyed a 15-year romance with her college sweetheart Danny Mozes - the father of her daughter Samantha, 15, and nine-year-old son Charlie - until their split in 2003. She then fell for Christine Marinoni in 2004, and the couple has been engaged to wed since 2009.
Nixon, who also has an 11-month-old son, Max, with Marinoni, is convinced her change in sexual preference was just that - a preference - because she considers herself a heterosexual who simply fell in love with a woman.
Her assertion has irked some gay rights activists, who insist being gay is a compulsion - but Nixon is standing firm in her beliefs.
In a candid interview with the New York Times Magazine the actress says, "I gave a speech recently, an empowerment speech to a gay audience, and it included the line, 'I've been straight and I've been gay, and gay is better.'
"And they tried to get me to change it, because they said it implies that homosexuality can be a choice. And for me, it is a choice. 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.
"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. I say it doesn't matter if we flew here or we swam here, it matters that we are here and we are one group and let us stop trying to make a litmus test for who is considered gay and who is not.
"Why can't it be a choice? Why is that any less legitimate? It seems we're just ceding this point to bigots who are demanding it, and I don't think that they should define the terms of the debate.
"I also feel like people think I was walking around in a cloud and didn't realise I was gay, which I find really offensive. I find it offensive to me, but I also find it offensive to all the men I've been out with."