Well, Natasha in Sex and the City was sort of an obstacle in Carrie and Mr. Big's on - again - off - again relationship. She was a part of Mr. Big's more 'grown - up' phase where he tried to settle down. But Carrie's presence always loomed over their relationship. Natasha was the kind of character who seemed to have it all together on the surface, but the turmoil in her marriage showed that things were not as perfect as they seemed. She was a symbol of the more traditional relationship that Mr. Big was attempting, but his heart still had a soft spot for Carrie's more free - spirited and somewhat chaotic world.
I'm not sure specifically what a 'city sex story' is as it could be many things. It might be a fictional story set in a city that involves some aspects of relationships or experiences related to sex, but without more context it's hard to say more.
The 'Sex and the City 2 Story' is about the four friends exploring a new culture in Abu Dhabi. Carrie is having marital woes, like communication problems with Big. Charlotte is trying to be the perfect mom while also maintaining her own identity. Miranda is stressed about her job and family. Samantha, being Samantha, is trying to have fun and be herself, but often gets into trouble because of the cultural differences in Abu Dhabi.
Yes, 'Sex and the City' is based on a true story. The creator, Candace Bushnell, wrote a column in The New York Observer which was inspired by her own experiences and the lives of her friends in New York City. It was this column that served as the basis for the popular TV show and subsequent movies.