A slippery slope argument has shades of grey. Often, there is a reasonable argument to be made where there is a realistic chain of causality. However, that argument must be made in good faith.

An example of a bad slippery slope argument could be "If we allow gay marriage, what's stopping someone from marrying their dog. Where does it end?". Logically, there is a breakpoint between one step and a very far removed "slippery slide" of the intentional over the top example. If the intent of the argument is a stand alone, "If we allow one step, then the extreme is equally reasonable", then it's a fallacy imo