Retna Digital

Cynthia Nixon is making it official with her girlfriend of nearly six years, education activist Christine Mariononi.

The "Sex and the City" star, 43, showed off her diamond engagement ring at the Love, Peace and Marriage Equality rally for gay rights in New York City Sunday. The event supported New York governor David Paterson's proposed bill legalizing same-sex marriage.

Nixon has said she would wed "if it became legal in New York. "I don't really want to get married to get married pretend. I think we'd like to do it in a real, actual, legal way that the state would recognize."

The actress is mom to Samantha, 12, and Charles, 6, from her prior relationship with English professor Danny Mozes.

Nixon is set to star in the "Sex and the City" sequel, out next spring.