Cynthia Nixon did it. Lindsay Lohan's doing it. TV shows are based on it. Is it our imaginations, or are wives and girlfriends ditching their men and falling in love with other women? New science says that sexuality is more fluid than we thought. Lately, a new kind of sisterly love seems to be in the air. In the past few years, Sex and the City's Cynthia Nixon left a boyfriend after a decade and a half and started dating a woman (and talked openly about it). Actress Lindsay Lohan and DJ Samantha Ronson flaunted their relationship from New York to Dubai. Katy Perry's song "I Kissed a Girl" topped the charts. The L Word, Work Out, and Top Chef are featuring gay women on TV, and there's even talk of a lesbian reality show in the works. Certainly nothing is new about women having sex with women, but we've arrived at a moment in the popular culture when it all suddenly seems almost fashionable — or at least, acceptable. Statistics on how many women have traded ...