Two other things: I also think that Trek should have had a few homosexual relationships, but dealt with in a mature way, not as a silly Mirror Universe titillation.
And, I love the fact that Trek likes to feature unexpected pairings and interspecies relationships, and hybrid children - even if it doesn't make sense scientifically - since it is really a metaphor for interracial/crosscultural relationships, overcoming differences etc.
Devileyes is right.
Gays, when depicted at all, are present in the story because they are to be 'the gay charactor.'
Advertising prior to the episode Rejoined leaned heavy on the 'gay kiss' between Terry Farrell and a guess star Susanna Thompson. TNG episode Outcast was to be the much anticipated gay story fans had asked for, it wasn't.
Jadzia Dax would have been the perfect long term gay charactor. She was more open than most of the people on the various series. The slow to build romance she had with Worf could of been written as same-sex. If Tasha Yar had lived, she'd of been one possible choice. Or a brand new charactor could of been created, but not a 'gay' charactor.
James Kirk; "Crewman, what are you doing?"
Guest star: "Just being gay, sir"