Why were there so many bisexual heroes in the 1970s (including Robert A. Heinlein's Lazarus Long), and was bisexuality viewed as "futuristic"? More to the point, why did bi heroes mostly vanish throughout the 1980s and 1990s, and what brought them back? Erotic science fiction author and Circlet Press publisher Cecilia Tan explains.
Tan has an indispensible history of bisexuality in science fiction, over in Lambda Literary. She talks about the rise of bi characters in the 1970s, and the recent spate of bi heroes, including Captain Jack on Doctor Who and Torchwood. Here's the relevant part about the 1970s:
Lazarus Long went bi at the same moment in popular culture when bisexuality was equated with avant garde–perhaps futuristic–social attitudes. Bisexuality as an identity or orientation existed so far outside the monosexual "norm" that David Bowie's public image was cemented as not only bisexual, but alien from outer space. (Bowie would much later explain in interviews that he latched onto bisexual identity as a way to shock the mainstream and give himself street cred, but despite a well-documented dalliance with Mick Jagger, he eventually settled into lifelong heterosexuality.) Science fiction literature of the 1970s used bisexuality as a signifier of outsider/other, alien, or futuristic status. Samuel R. Delany's Dhalgren (1975) uses the sexuality of "The Kid" as well as many other narrative elements to dislocate the reader from the status quo. Many of Marion Zimmer Bradley's characters in her Darkover books are bisexual, in particfular the Darkovans (as opposed to the Terrans from Earth, who are more "like us.") Joanna Russ's The Female Man was written in 1970 but published in 1975 and intertwines elements of bisexuality with deep thinking about gender and gender roles. Another thinky example from the time is David Gerrold's The Man Who Folded Himself (1973), where the homoerotic aspects enter with the idea that time travelers can have sex with themselves. Even the not-so-thinky but just as subversive Rocky Horror Picture Show (1975) runs in this gestalt track of bisexual-as-other.
The whole thing is well worth reading — did you know that George R.R. Martin wrote a novel with a bisexual female main character (Nightflyers), but it was made into a movie where she was "straight-washed"? [Lambda Literary]