Counteragument #16: First homosexuality. Now incest. What's next, bestiality?
Rebuttal: Bestiality does not occur between consenting humans, who understand how to communicate with each other (and if they are unable, then it's not consensual). This is not even on the same mountain.
You cannot appeal to the slippery slope fallacy when your argument exemplifies the validity of certain slippery slope arguments. "Slippery slope" is not automatically a fallacy if there are good reasons for believing that one thing (a) will lead to another (b). Because during the debate over homosexuality (a), incest was suggests widely as a logical eventual consequence (b), you're standing on the very slippery slope that you aim to dismiss from your opponents arguments. Contextually, it's ridiculous.
So the questions become:
Are the valid slippery slope arguments against your stance that lead to other outcomes that we can agree would not be desirable?
Is your specific suggestion of a slip, bestiality, a valid slippery slope argument?
I'll handle (2) first:
You provide and apples-and-oranges argument against bestiality by appealing to consent. However, the consent argument cannot be made to preclude all forms of bestiality.
Let's take a short detour into what we mean by consent. First-order desires, legally, are the basis of consent. Did I want to have sex? Did I not? Second-order desires don't enter into it. So, for example, I cannot claim that I was raped because "I wanted to want something other than sex (like to instead spend that time studying), even though I did want sex at the time." This is meta-reflective upon one's own desires. Consensual sex interfering with studying is not a case for rape, obviously, because its the first-order desires which are relevant.
We can say that a dog can act in accord with its first-order desires: it is capable of acting in accord with its wishes. Those wishes may be simple, and they do not include the meta-reflective capability to consider its second-order desires, but those are irrelevant toward consent. So, you can see where this is going: a dog is capable of consensual sex, so long as it is acts in accord with its first-order desires. There are forms of bestiality where the animal does, in fact, act in order to its first-order desires. Those forms of bestiality include, for example, a male dog mounting a female human. If the animal's first-order desires are reciprocated by the human, this is consensual sex.
Your slippery slope argument fails on its proposed slip. As there are types of bestiality that succeed, nevertheless, under appeal to consent, we have a valid slippery slope argument. Counterargument #16 fails.
To address (1) now:
Because your argument fell on (2), it falls automatically to (1). There is no need for a more generalized dismissal.