I'm wrapping up a PhD in a biological field and while procrastinating writing my dissertation, I got into an argument with some lab mates about genetic manipulation of people.

TL;DR - I think that, not only should this be allowed, it should be encouraged.

Longer explanation: The earliest examples of genetic manipulation that are likely are things like treating genetic disorders (like cystic fibrosis, or a recent example in the news, mitochondrial disorders). Some object even to this type of manipulation on fears of eugenics. This does not strike me as a particularly forceful argument since we can aim to prevent diseases without advocating killing people that have them. This strikes me as akin to saying we shouldn't vaccinate against polio for fear that people with polio will be considered subhuman.

Another objection is that unborn children can't consent to alterations of their DNA. This also doesn't strike me as compelling, since children can't consent to anything in their genome. Just because it's been an accident for 3 billion years doesn't mean that's the best way to do it.

Another objection is the slippery slope towards designer babies and the persecution or discrimination of unmodded people (the GATACA problem), but this is an issue of discrimination laws, not a necessary outcome of the technology.

This is somewhat similar to the objection that only the rich are going to be able to afford it, and that will increase class disparities. This is the argument that I'm most sympathetic to, as I believe that wealth inequality is a major problem. But we need to solve that problem whether or not we use genetic engineering on people.