Robert P. George makes a case for limiting marriage to heterosexual couples. One of his arguments is the logical (as opposed to the causal) slippery slope. It goes like this:
1. There is no principled basis for limiting marriage to two individuals if homosexual "marriage" is allowed. (In other words, homosexual and polygamous "marriages" stand or fall together.)
2. It is unacceptable to allow more than two individuals to marry. (In other words, polygamous "marriages" fall.)
Therefore,
3. It is unacceptable to allow homosexual "marriage." (In other words, homosexual "marriages" fall.)
Since the argument is valid, those who reject 3 must reject either 1 or 2. Some people bite the bullet and reject 2. They believe that polygamous ("polyamorous") marriages should be allowed. I have seen no good case for rejecting 1. Can you think of a principled basis for allowing homosexual "marriage" but disallowing polygamy?
Addendum: The causal slippery slope has the following structure: "If you do or allow X, which is not bad in itself, it will cause Y; Y is bad; therefore, you should not do or allow X." The logical slippery slope has the following structure: "If you do or allow X, consistency will require that you do Y; Y is bad; therefore, you should not do or allow X." See the difference?
Keith Burgess-Jackson, J.D., Ph.D. 