The problem with putting gay characters in games is that the developer would probably feel the need to make sure we KNOW they're gay, whereas if they're straight the sexuality of the character would probably be completely unremarkable.
I have no problem with playing a game where the main character is gay, but I don't see how it could be incorporated effectively into the story without seeming like pointless pandering. And if it can't be incorporated into the story, what's the point?
In Half Life we assume Gordon Freeman is straight even though he never says a word, just because Alyx seems to be interested in him. In The Darkness Jackie is straight and has a girlfriend. Would it make a difference if that were a boyfriend? To some people, maybe. In Halo the Master Chief doesn't really have a sexual identity unless you consider his bond with Cortana to be some kind of platonic love - of course there's novels where he supposedly has a girlfriend in the form of a Spartan named Linda, but I don't consider the Halo novels anything more than fanfic anyway.
Then there's Gears of War where Marcus and Dom share a barely suppressed bromance, complete with Brokeback Mountain references in the co-op achievements. I'm sure the inclusion of Anya as a squad member in Gears 3 will result in Marcus/Anya romance, but I secretly hope they make Marcus/Dom canonical just to generate fanboy rage.
Any romance storyline in a game needs to be relevant to the story. Most games don't have romance storylines, and a large number of the ones that do allow you to choose if your character pursues a homosexual relationship if you want. Dragon Age, Fable 2 and 3... sure, Mass Effect doesn't, but there's no real need for Manshep to pursue Garrus or Femshep to pursue Tali. You could do it if you wanted, I guess, but does it make sense in the story for Garrus to be receptive to Manshep or Tali to Femshep? Is it necessary?