Slippery slope?

Let's just take this statement (this is where we find the fallacy):

Now that gay marriage is legal in the United States, next to be legalized will be polygamy, incest, pedophilia, then necrophilia.

The following two questions are good philosophical questions to ask and should be discussed.

This example has been used quite often as a classic slippery-slope fallacy, and rightfully so. I think mys standard explanation at Slippery Slope explains this well. In short, the slippery slope is actually a series of weak analogies. The claim is that gay marriage is like polygamy. This claim can certainly be argued that it is not in many ways (e.g., it has been empirically demonstrated that polygamist cultures experiences far less well-being). So right there the chain breaks. Clearly, there is a much bigger problem with incest and pedophilia where serious harm is being caused, making them actual moral issues.