Stacey, its comforting for some to think things are solid and immutable, even when they are not. There are ALREADY multiple definitions to work with. For example, if you live in Alabama, you can marry your first cousin. If you live in Arkansas, you cannot. Where is this mythical single definition? IN several European nations, you can marry a sibling, with some restrictions. And then of course, in many nations and in some religions, marrying more than one person is perfectly OK, if not expected.