JF: I disagree. A slippery slope is “only” a slope as opposed to a free-fall drop because there indeed are differences between different points on that slope, like one you’ve pointed out here between “gay marriage” and bestiality. But the slippery slope is very much valid, just not a straight (no pun intended) drop down.
But, even by your logic, if your sole criterion is “people should be able to marry– and be married to– whomever they choose” then incest (and polygamy – why limit to one?) should also be valid marriage material.
Regardless, the “slippery slope” is indeed a very big concern.