I might answer this from an FPS perspective.
In FPS, immersion is often the case, more so than in any films. The developers want you to believe that you are the character in the game. You are gordon freeman, you are Master Chief etc.
Now if that character was gay, would you feel a little squeemish? Maybe it's just my instinctive homophobia speaking, but I would feel rather odd hitting on male characters. However I wouldn't feel odd having male characters hit on me. I don't mind playing characters who have lore behind them about them being gay, but once my character starts making passes at other characters, I would probably draw the line there.
My point being, in films or books, you are reading about a character, you might identify with them on certain levels, but you still are at an arms length watching from a different perspective. It isn't as awkward to watch a movie about a gay main character, Where as in an FPS you are directly in the controlling seat, making it awkward (at least it would for me).
So therefore if my character started hitting on characters without my control who were male, I wouldn't really enjoy it so much from the immersion standpoint. And so I perhaps wouldn't purchase the game. And as sexuality is not a key part of gaming, I suppose not many game companies are willing to take the risk of going for a particular demographic?
As for the other genres, sexuality just isn't as important in gaming, as gaming is about blowing the crap out of each other, or jumping on clouds, collecting coins. Not hitting on people.
How much of perceived lack of gayity is just as a result of assuming the characters you play are heterosexual?