George's views about embryo research (and abortion) appeal to the same kind of "kind" argument: embryos are the "kind" of being that has a "rational nature." Yet embryo's are not rational, or not yet rational, and so this view seems problematic also. Years ago I wrote this paper discussing George's arguments. I re-post it here in case reading it would improve discussion of each "kind" argument, about embryo research and marriage:
Critical Study of Robert P. George and Christopher Tollefsen, Embryo: A Defense of Human Life
Abstract: In Embryo: A Defense of Human Life (Doubleday, 2008), Robert P. George and Christopher Tollefsen argue that human embryo-destructive experimentation is morally wrong and should not be supported with state funds. I argue that their arguments are unsuccessful.
I never really tried to publish this paper because I found this book a few years after it was published and so by that point I thought there wouldn't be much interest in a longer critical review anywhere.
For discussion of similar arguments, see my "Abortion, Metaphysics and Morality: A Review of Francis Beckwith’s Defending Life: A Moral and Legal Case Against Abortion Choice."