Answering this objection is quite simple because no one ever sets up the argument properly to begin with. It goes like this: "Accepting transgenderism will inevitably lead us to accepting horrid
ideas; because those ideas are wrong, transgenderism itself must also
be wrong." The horrible things at the end of the slope are rarely articulated, instead left to the hearers imagination. When they are mentioned, the common fears are homosexuality or sexually
deviant behavior like marrying your houseplant. The problem with this
argument is that is relies on the assumption that accepting one idea
will lead to the other but that assumption is never explained. Take the article entitled Allowing Children to Choose Gender Is a Slippery Slope or Continuum or Slippery Slope. Notice the lack of explanations. If the argument is never explained, well, it's no argument at all. Claiming a slippery slope exists does not make it so.
If someone did articulate the idea, which I have yet to actually see, they could do it one of two ways: They could argue that transgenderism is definitely and inevitably lead to bad things. This is impossible to make since there are plenty of people who believe in transgenderism without the horrid things. There are even homophobic trans people! The inherent diversity of transgenderism makes inevitability impossible.
The second way is to suggest that transgenderism will make it easier for other things to come. Using homosexuality as the example again, this definitely happens. Homophobic trans people exist but they are a minority. Unfortunately for those making it, this is a logical fallacy and not a valid argument. Take an example from the early Church. Jews were becoming Christians and their sins were freely forgiven. This led some converts to believe they could sin all they wanted since everything was forgiven. Applying our argument, a Jew argues that Jews should not convert because they (or at least some) will sin more. Does fault lie with grace and transgenderism or does it lie with people's personal conclusions? Paul is clear.
Further, all slippery slope arguments fail because they are speculative. Yes it is possible that embracing transgenderism could lead to embracing polygamy. Anything is possible. Better to ask, "Is this likely?" I have never seen an attempt to show this is likely. There are no statistics that people who embrace transgenderism embrace polygamy. However, the opposite exists! People who favor polygamy unanimously favor the gender binary. Indeed, Utah is the least transgender friendly place in the United States! Next time you think or hear about trans people and pedophilia, bestiality, and so on, simply ask, "where's the evidence?"