nothing against gay guys but i dont see a purpose in making a gay protag. if a player wants the protag to be gay then he can Imagine that the protag is gay if that is what he would like to so inside of his little fairy mind.
Well, if you want him to be straight you can imagine it inside your little bigot mind.
Considering how little sex plays into the game (not really part of the plot, a token part of gameplay) it's really not a big issue. If this was a dating sim then yes you would want a little more diversity. Since most of this game's audience is straight there really isn't any reason to have this unless Rockstar wants to be edgy.
The player should have the choice to bang whatever gender NPC they choose to, therefore allowing the player to determine the character's sexual orientation.
I don't think it would make a difference to people if the character is gay as long as it is incorporated well into his character and his story, and as long as the game-play works. People will complain regardless, but if a game is good, people have a tendency to shut up and play it, even if they do go online to whine about it afterwards.