The Thin End of the Wedge
But really, life would be so much easier if people would just judge each case on its merits, not worrying about what might happen if the chain of dominos falls.
For example, take a look at (picks hot button political topic at random) abortion - the most radical opponents think that even allowing it in cases of rape or other equally barbaric situations is wrong. Give in on that and pretty soon "those people" will want more and more concessions and we know where that ends up.
The most radical proponents figure that even if the baby is abseiling its way out of the uterus with equipment provided by REI it's still fair game for early euthanasia - just in case it's inconvenient to somebody somewhere.
Try to pick a position in the middle and both sides throw rocks at you. Which is why the extremists get to fight the battles and common sense goes out of the window.
While we're on volatile topics, I often have really interesting conversations with my carpool buddy on our 45 minute drives. Because we've been driving together for about 5 years we can talk about stuff we disagree on without getting all wound up about it. The other day we were talking about liberal and conservative positions on life and death and ended up pondering the following opposite points of view.
Conservative - save unborn babies, execute murderers.
Liberal - execute unborn babies, save murderers.
Hmm, which one of those looks wacked?