"I don't think we're seriously looking at a world of only girl children just yet, but I do think that when philosophers start talking about using medical technology to achieve things that aren't about health, so increasing people's IQ or life expectancy for example, you have to ask why we shouldn't all be girls," he said.
Why do you have to ask that?
"There are significant restrictions on the opportunities available to men around gestation, childbirth, and breastfeeding, which will be extremely difficult to overcome via social or technological mechanisms in the foreseeable future. Women also have longer life expectancies than men," he said.
Increased life expectancy would be nice, for sure. I'm not so sold on childbirth, etc., especially when you factor in the whole menstruation thing. (Sounds more like a net negative, if anything.) So it's not at all clear to me that being a particular sex is an advantage in the way that increased IQ, life expectancy, etc., are. (I would wish high IQ on my future children; I wouldn't wish any particular sex on them -- I'm sure they could do perfectly well with either.) Am I missing something?
Apart from intrinsic costs and benefits to selecting a particular sex for one's child, there's also the social milieu to consider. The more of one sex in the general population, the greater the benefit to being of the opposite sex (assuming we haven't also designed out heterosexuality). Some in China are just beginning to notice this.
Finally, apart from individual costs and benefits, we might consider the interests of humanity at large. See 'Gender as Cultural Specialization'. Even if women do better on average, the greater variability in men might make their extinction a notable loss to society at large (assuming the benefit of having more geniuses outweighs the costs of all those delinquents). What do you think?