I think people that are against it are worried that they'll have people come up to their character and force them into a relationship kind of like how sometimes in Oblivion/Fallout3 a character will run up to you and kind of force a new mission on you that you don't want to do.
And I think some people are worried that they'll put gay characters in the game and they'll have a Bioware situation on their hand where they're constantly getting verbally assaulted by horribly written stereotypes and dragged into a gay relationship kicking and screaming, ruining their perfectly heterosexual little fantasy world.
But the more I think about that the more I doubt Bethesda will put in gay characters. I mean, they had just 2 guys in F3 who MAY HAVE BEEN gay and I don't think there have been any real gay characters in Oblivion (Except Curio in Morrowind). And like I said earlier, I'm afraid that if they do do it they'll only make 2 pretty-boy human characters out of the entire game world and make one a rich lisping queens obsessed with clothes and the other a whiny momma's boy, neither acting anything like a man should.
Sorry, ranting here. As a gay dude who plays games, I'm really frustrated by the state of gay characters in games and even more frustrated at developers like Bioware who get a big, fat F in my book when it comes to gay content.
But Obsidian did a good job with the gay characters in New Vegas. Let's hope some of the writers from Bethesda were taking notes.