Just like any other form of evil, once the gate is opened ever so slightly it becomes nearly impossible to close.
I am sympathetic to the slippery slope argument. I also understand that medical science can cure disease, and while I certainly would draw the line at destroying one embryo to help another, I’m not sure I would oppose any genetic manipulation, simply because allowing some genetic manipulation could open the door to other manipulation.
However, because we have fallen so far, and because there is no real respect for life, I would tend to oppose even life-saving genetic manipulation, because at this time in our history, humanity simply cannot be trusted to act ethically.
There were people who applied this concept to the advent of antibiotics too. If you were dying of an infection in the natural course of things, then obviously that's what God wanted, and nobody should interfere with artificial, human-created things like anitbiotics. Some religious fanatics still apply this concept to ALL medical care -- like the nuts who recently let their young daughter die of diabetes, refusing to do anything but pray over her and spoon a little broth into her mouth as she lay dying for days. Yep, they stayed off the slippery slope of taking action in life-or-death matters, so the girl is dead, and the smug parents are proud of themselves for standing by and not interfering while (in their sick minds) God chose to kill her.