There are gay characters in the ongoing post-series novels.
The TV shows really dropped the ball. TPTB were far more interested in "playing it safe" and making as much money as possible than to"risk" adding a gay character that might upset some backward hicks and bible thumpers.
The shows are part of a business, and making money is the first (and, often, second and third) priority of any business, dude.
I would applaud any effort to cut the so-called "backward hicks and bible-thumpers" down to size but, from a corporate standpoint, they include scary people with heavy-duty influence and connections. Confrontation with such would probably cause lots of problems (and losses) with little or no real benefit to be had for the business.
Also noting, should characters have been brought into the shows just because they were specifically gay? Think on that for a bit. And how could this be done without pushing the dreaded STEREOTYPE
Understand, I am most definitely not anti-gay, and have always seen acceptance of other people's differances as a core value of Trek. But, simply asserting that "Come what may, Star Trek should have ....
" is (IMO) oversimplifying the entire issue.