Are slippery slope arguments always invalid?

It depends on the slope. Think slavery. The legalized slavery of Blacks, by Whites, does not bode well for any non-whites. "Today it's blacks, tomorrow it's Mexicans!" This is valid. As for the argument that allowing gays to marry will turn into people marrying fruit trees or animals, I call bull shite. So sometimes the slope is slippery, but not always.