See here for a New York Times story about polygamy. Many people are concerned that if two men or two women are allowed to marry one another, there will be no principled way to prevent polygamy. (This is known as a “logical slippery slope,” as opposed to a “causal slippery slope.”) They reason as follows:

1. Polygamy is unacceptable.
2. There is no morally relevant difference between polygamy and homosexual marriage.
Therefore,
3. Homosexual marriage is unacceptable.

The argument is valid, so there are only three replies to it. The first is to reject the first premise, i.e., to hold that polygamy is acceptable. The second is to reject the second premise, i.e., to identify a morally relevant difference between polygamy and homosexual marriage such that polygamy is unacceptable while homosexual marriage is acceptable. The third is to accept the conclusion.

Here’s another way to look at it. The following propositions are inconsistent:

1. Polygamy is unacceptable.
2. There is no morally relevant difference between polygamy and homosexual marriage.
3. Homosexual marriage is acceptable.

Everyone must reject at least one of the propositions. Which do you reject? If you reject 2, please state the morally relevant difference.