it's a game about how you're the one person who is capable of stopping the destruction of the universe. there's no reason for there to be an option to join the reapers, as that'd be an entirely different game. the grey area in this story is how you go about stopping them, which honestly could be a lot more complex and affect the outcome of the game. but still, you do make choices that make a difference, and from what I understand ME3 will be even bigger on building on your earlier choices. you can't ALWAYS expect every single option to be available to you in a game. it's never been done before, so why go around expecting it from games where it doesn't even make sense?