Slippery Slope
Conclusion based on the premise that if A happens, eventually through a series of small steps, through B, C.....X,Y,z will happen, too. IF we don't Z to occur A must not be allowed to occur either.
Examples
If you get a B in high school, you won't get into the college of your choice, and therefore will never have a meaningful career.
"If we legalize marijuana, then more people would start to take crack and heroin, and we'd have to legalize those too. Before long, we'd have a nation full of drug addicts on welfare."