The assumption here is that there is no set of genes that are guaranteed to be purely negative for humankind. That's just false. There's just no reason we'd want to let somebody grow up with cystic fibrosis, for instance.
Yes, except being heterozygous for the cystic fibrosis mutation actually turns out to give resistance to cholera, in the same way that being heterozygous for sickle cell gives resistance to malaria.
The sickle-cell anemia vs. malaria case is actually unusual, and a population high in sickle-cell anemic individuals is not actually a desirable outcome.
The former is false, the latter is true. The only reason something as disadvantageous as sickle cell has existed to be passed down through the generations is, and I cannot stress this enough, malaria is such a horrible disease (and thus a good natural selector) that such a disadvantage can be outweighed by its survival advantage in heterozygous form. There is NOTHING unusual about that situation; if it was unusual 200 years ago, sickle cell as a trait would have quietly died out.
Also, if we can prevent genetic engineering, then surely we can prevent choosing the gender of children. If you can't prevent choosing the children's gender, then how do you think we can prevent other genetic engineering?
You can't. Once the Pandora's Box is open to the rich, enough money can not only get the genetic meddling done, but keep the appropriate parties quiet. But just like you don't get rid of murder by outlawing it, it doesn't mean society should give a green light to weeding out diversity from the human genome based on what popular culture believes is attractive and beneficial today.