Slippery slope arguments are for the most part nonsense, used when there is no rational argument against a position except to say if you do this imagine what could happen next. It was once unthinkable that there couldn't be slavery; make a slave free and the next thing is that he will want the right to vote. Allow divorce and society will collapse. Permit birth control and the purpose of marriage will collapse.Permit gay marriage and the next you know bestiality will be legal.
In many instance making the unthinkable normal is called civilized progress.