Arthur C Clarke was gay. Did he portray any of his characters as gay (or otherwise LGBT)?
In Rendezvous With Rama (1973) two male crew members are in a polyamarous relationship with each other and a woman back on earth.
But no one can predict where the lightning will strike, and years ago Mercer and Calvert had established an apparently stable liaison. That was common enough. Much more unusual was the fact that they also shared a wife back on earth, who had borne each of them a child. Norton hoped that he could meet her one day; she must be a most remarkable woman. The triangle had lasted for at least five years, and still seemed to be an equilateral one.
Duncan Makenzie, the protagonist of Imperial Earth (1976) is a bisexual man, and sexual encounters with both genders are described. In fact, a romantic/sexual triangle between Duncan, his friend and sometimes lover Karl, and a girl called Calindy, is central to the plot.
George and Jerry who appear in the third book of the Space Odyssey series 2061: Odyssey Three.
An old gay couple who are two of Floyd's longest and closest friends; they had worked in the arts in earlier life. George and Jerry help to manage Floyd's personal affairs while he is away.
Walter Curnow from 2010: Odyssey Two is described as being a "cheerfully well-adjusted polymorph" - i.e. happily bisexual.
Yes, in The Songs of Distant Earth, pretty much everyone on it is, at least, bisexual.
In it, Arthur C. Clark treats sexuality has something volatile, with people not belonging fully to one end of the scale (homo or hetero).
The colony on the planet in which the story takes place (Thalassa), was founded with the aid of machines, one of which containing all human knowledge and heroic, or good, history. Wars, Crimes, Religion and overall bad notions, acts or concepts were left out - the concepts of possession, racism, discrimination and jealousy among them. So regarding relationship and sexuality, this resulted in a society where there was no norm for either relationships or sexual orientation. Everyone was free to be themselves in those regards.
At some point the author explains that Earth society came to accept sexual orientation as not set in stone, but having a tendency percentage. An example in the story is Loren, a man for Earth, which identifies as an heterosexual (but with a small tendency for homosexuality), is married to a woman and although in love with Marissa, has a one night stand with her brother, Kumar. Kumar, on the other hand is mentioned as being known for having partners from both sexes.
There's a rather explicit dialog concerning bisexuality in "The Songs of Distant Earth", right at the beginning of chapter 35 "Convalescence":
"I'm quite surprised," Loren had interjected at one point in a rather seamy narrative. "I'd have sworn you were ninety percent hetero."
"Ninety-two, according to my profile," Horton said cheerfully. "But I like to check my calibration from time to time."
The lieutanant was only half joking. Somewhere he had heard that hundred percenters were so rare that they were classed as pathological. [...]