Gender selection?

Why do some people get so upset at the idea of gender selection (like the Shettels Method)? I think everybody knows that there are no guarantees that it's going to work, and just because someone is hoping for one doesn't mean they won't love the baby if it is the other gender. It's not like you are selectively aborting children if they aren't the right ! I read a post from one guy who said it was messing with what nature intends......but so is birth control and no one has a problem with that. Why are people so against it?