Laws can easily be enacted to prevent designing babies to select superficial traits such as eye color or hair length, therefore cancelling out most of the arguments opposition has put forward. This is not the intended purpose of this technology.
The term 'designer baby' in medicine usually refers to solving a medical problem, such as Tay-Sachs as mentioned.
If you think we are going to design our babies to what we want what is the point of having a baby it's all about "oh i want my baby to be pretty" or "i want my baby to be skinny" you are supposed to love you kids no matter the size no matter the beauty no matter how mean they are they came from you and your wife/husband and thats what came out if you don't like your baby stop making them.
People may say we already alter genes in animals, plants, and we select people naturally, such as tall, skinny people. Animals that are genetically engineered, such as cows to produce the best beef, plants for the biggest tomatoes, and so on. But those are for consumption purposes. These are not our children we are talking about here. And while most of us do try and select mates who are, for example, tall and skinny, genetics is influenced by the genes of the parents, but not completely. To an extent it is random. When we actually pick and choose our childrens' genes, we are reaching the point when we are playing God. And although I agree there are some instances where you may want to get rid of a gene, for, say, Tay-Sachs, a fatal disease which probably also makes the child suffer throughout their short life, that's different. Getting rid of a disease is a benefit to the child, as they will be spared from suffering. This, on the other hand, is for cosmetic reasons alone. These "designer babies" are created simply so a parent can boast they have a "perfect child": muscular, smart, beautiful. Other than giving a child cosmetically "better" genes, it does nothing, and, in fact, it may even embarrass the child, knowing their traits are not genuine, and may make them feel not unique. Also, when we say, "Well, it could lead to better behavior." Really? This may sound like an odd allegory, but raising a child is honestly not that different from raising a pet. While you don't train your child to "sit" and "stay" and "roll over", you do teach them to be good kids, and have manners, and so on. What happens if you're a good trainer? You get a good dog. What happens if you raise your kid well? You get a good kid. You don't just "inherit bad manners", it's the way you are raised. Not to mention that this would lower the gene pool, which is dangerous for a species.
Oh, and by the way...Hitler, anyone?
This is unethical in every sense of the word.