Here's a common challenge on the issue of marriage:
One of the standard arguments provided by the homophobic right against gay marriage is a form of what we call the domino fallacy or the slippery slope argument. The idea is that if you take that seemingly innocuous first step, it automatically leads to a second, which will force a third, and so on and so on until, next thing you know, there we are in Satan's own livingroom listening to Yanni on 8-track. In the case of marriage, the argument goes, if we open up the institution to same-sex couples, then we will be forced down the slippery slope to include all sorts of unusual couplings including cross-species arrangements.
How would you respond to this one? Let us know in the comments, then look for Alan's answer to this challenge on Thursday.
(Incidentally, if you read the rest of the post linked above, you'll find that the author commits the fallacy of begging the question by defining marriage in a way that "proves" opposing same-sex marriage is bigotry. But of course, it's the definition of marriage that's the very thing in question.)