Logical Fallacies - Slippery Slope

I have lived in areas with freezing rain.  You people who have only experienced Rocky Mountain snow do not know what it's like.  The rain falls, and instantly freezes on anything it touches.  That releases a bit of heat which makes a slick layer of water on the top.  Friction practically goes away.  If you start moving down a slope, there is really no way to stop.  This concept forms the imagery of an interesting logical fallacy, the slippery slope.

It goes something like this.  If I take the following action, I will inevitably continue to take more and more extreme actions, so I should not take the first action.  If you start eating cotton candy, soon you'll be eating it every morning.  After that you'll progress to eating cotton candy for every meal, and become malnourished.  Therefore, you should never eat cotton candy.  In this context, it's pretty clearly a fallacy.

Take the following example:

If we allow gay marriage, where will it stop?  We will move toward allowing polygamous marriages, and perhaps marriages between siblings.  Even parent-child marriages!  Abominations in the gene-pool, and the destruction of the human race as we know it!  So we can't allow gay marriage!

This should be seen as just as ridiculous as the cotton candy example.  Now look at this one:

If we allow anyone to refuse service to a gay customer, where will it stop?  People will start by refusing to take pictures at a gay wedding, and pretty soon gay people won't be able to shop at the supermarket.  We can't let this happen!

The amazing thing to me is that many of the people who readily recognize one of the above examples as clear fallacy often entirely miss that the other one is based on the same logic, and just as unsound.  These fallacies are meant to rile up the masses -- they are a call to arms.  I think we need to stop this kind of verbal warmongering and start actually conversing.  That way we can problem-solve and actually come out better in the end.