Slash fiction — a branch of fan fiction that imagines straight heroes getting together — is strangely popular among straight women. Helen Joyce examines the light it sheds on female sexuality. — By Helen Joyce It all began with “Star Trek”, or more precisely with James T. Kirk, the captain of the Enterprise, and his first officer, the half-human, half-Vulcan Spock. The show, which debuted in 1966, was no immediate hit: it was nearly cancelled twice before finally being taken off the…