Like I asked before, how do we know that people won't be screened for "bad" genes and prevented from marrying/procreating/holding a job because of the perceived flaws in their DNA?
Well, this is something we could perfectly well do even if we didn't exercise our ability to change genes. That just involves looking at the genes. We can look at the genes now, we could look at the phenotypes since forever. There was a time in the USA when we did forbid people to procreate (and sterilized them) based on phenotype. There was a time when Spartan fathers tossed their sickly children off a cliff.
The bad face of eugenics can appear whether or not we can give ourselves better genes through technology --- but, if you think about it, it's much
less likely to appear if we have the technology. Imagine some politician, or some dictator, saying "We must stamp out cystic fibrosis by screening everyone and preventing carriers from marrying."
Which he could do now. But then imagine that we have such powers as are presupposed in your OP. Would his attack on liberty not seem, and be, perfectly futile, if carriers could simply elect to have children without the gene? --- which they would.
Isn't decreased genetic diversity a bad thing in itself?
That depends on the particular allele of the particular gene. Is there anyone who wants to stick up for phenylketonuria because we need diversity? Again, this is a case-by-case question.
Is anyone who brings up questions like this engaging in nothing but knee-jerk fearmongering, impeding scientific progress for no reason?
Well ... with respect ... it's the way you brought it up. Your talking point was: "If this happened, wouldn't it technically be ...
eugenics?" Which
is fearmongering, and is no reason, or no good reason, to impede anything.