I think this is a very personal bit of information that is not necessary to be collected officially by the government. If it could be optional, that is a different story. Not all people would be willing to share this information and therefore it would likely be inaccurate to some degree. Imagine someone who is afraid to come out as homosexual being forced to choose to state this on their census form or choosing instead to lie about it. Information can be great and all, but it can also be used negatively to discriminate.