Yeah, the Christian objection to gay marriage in the US has always been that it's a slippery slope that OBVIOUSLY and INEVITABLY leads to legalized polygamy and people marrying their pets. If that sounds stupid, that's because it is.
That's bullshit.
They hate LGBTQ+ persons, its not about a slippery slope, they just make absurd arguments like that because their brains are broken and they think making such false equivalency will work on people that don't hate LGBTQ+ people simply for existing.
They will also alternatively claim that gays will lead to extinction of us as a society, or pedophilia, or any number of other arguments in an attempt to get people to hate gays just like they do.
Not only that, but as their regular defense/perpetration of pedophilia (or whatever the "techinical" term for old dudes trying to fuck young teenaged girls is) and calls for genocide and death penalty for any and all crimes and other psychopathy shows, they don't actually care about the slippery slope, they really simply only want everyone to hate gays like they do and will resort to any and all specious arguments when trying to galvanize such sentiment.