The principle I'm not sure we should be encouraging the principle of parents choosing what their children should or should not be taught. On this issue, at least, I can understand, even though I do not agree with, the position that these parents are taking: homosexuality is condemned in most religions. But on the other hand, should religious beliefs have a place any more in guiding our education system? And what if it applies to other aspects of education, eg science lessons? Would we be happy if the principle were extended to, eg, blocking children from learning about the Big Bang Theory, etc?
There's no basis for rejecting LGBT education from any perspective other than a (fundamentalist) religious perspective. No child is being indoctrinated into believing that they can make a choice to be anything other than what they are by teaching about it, even from a young age. Acceptance and inclusivity are healthy values to present to young people.