We've already lightened the pressure from our environment on our genes. With medicine and other advances, people who never stood a chance of survival can live now and even pass on their genes. Natural selection isn't able to mold us as easily now. Before, if you had a genetic defect that made your heart not function right, you would die but now we have people survive it. People with problem genes are going to pass them on and they will become more and more common and in some cases perhaps become worse.
This is why I think this kind of technology will become absolutely necessary the further on we continue. That isn't to say we shouldn't be exceptionally careful and I do agree that it will require some heavy regulation even once we are able to start. I'm certain that mistakes will be made but that the benefits far outweigh risks.