And therein lies the problem. Where do you draw the line and say "these people can be married, but these people cannot."? If you are going to change the definition to allow a new group to the list, you MUST show definitively why this one group is allowed to join the party that is not also applicable to other groups. This isn't slippery slope; it's unintended consequences. Eventually pedophiles will ask, and we will give them good reasons why they can't get married. We don't have good answers for why people can't marry more than one person, why a person can't legally have a concubine, or why someone can't marry their mom. At least, as of now, I have not heard a single argument as to why we should allow same-sex couples to get married that is not just as applicable to the previous three that I mentioned. In fact, many of the reasons I hear promoted so vocally would also be applicable to pedophiles, such as "people should be able to marry anyone they want."

And because this line cannot be drawn in an uncontestable way, it must be removed. Or the government must admit it has an interest in promoting the nuclear family above all other types of relationships, and the rest of us are not equal so tough shit.