It’s definitely a tricky line, and we need to be extremely careful, but I agree.
Stuff like that one gene that some people naturally have, that provides full HIV resistance, for instance. If we get to the point where it becomes easy to simply give someone that before their birth, why not, if there are no ill effects. At a certain point of medical advancement, the line almost begins to blur between adding natural resistance, and something like vaccines.
The same goes for as you mentioned, screening unborn children for genetic diseases. I saw a documentary a year or so ago, with a couple who had one child with an awful terminal illness, caused by a genetic diseases. The couple then performanced in vitro fertilization, and had 4 eggs fertilizated. Each were screened, and one was found to be healthy, without the genetic disease. So, that egg was used, and the couple had a healthy child. I took an anthropology class last year, about technological ethics. My professor had personally worked with that couple, and hearing her speak about their process made me tear up. Before they had such hopelessness, knowing that it was unlikely that they would ever have a healthy child, without just bringing more suffering into this world.
There’s definitely a difference between giving someone blue eyes and white skin, and avoiding genetic diseases and other health issues.
IMO the slippery slope argument is silly in this case, when there’s a decently clear line, and the benefits for humanity are so, so huge.
I don’t like to pull the gay man card very much, but I would give so much to not have to take PrEP, for instance. The side effects are rough for me, but it’s necessary for the long-term safety of me and my partners. Natural immunity to HIV would be incredible. Even with PrEP and other protection, there’s still always a chance.