You can't reason a person out of a position they haven't reasoned themselves into.
Not only that's an ad-hominem, I find it a somewhat poor one. I can assure you that at least I for one have no feelings or personal interest whatsoever on the topic. I don't even have a sister I could marry, mom is well past menopause, and generally the only even possible match is an ugly cousin.. who's already married anyway.
I don't see why it's necessary to "reason yourself into a position" or have any feelings whatsoever about it, to take part in an intellectual exercise. After all, I think most of us don't have any feelings for or against dowsing either, but here we are anyway. It's not any hatred of them or their job, it's just matching claims to evidence.
I don't see why the same standard shouldn't be applied to any other topic. Which brings me to...
I submit that, under law, prevention of harm is a perfectly legitimate reason to outlaw something, which is why we have things like mandatory safety equipment on airplanes and ingredient labels on food. And laws against child abuse, and laws against incest.
If there is actual provable harm done, let's hear about it. But speculations about feelings and other posters' feelings are not it.
Again, see dowsing. Some people obviously feel strongly about it, but that doesn't make it true. Or see religion. A majority of the world's population feels actually more strongly about atheism than about you being another religion, but that doesn't mean we should have apostasy laws. Likewise, just because some people have a strong feeling about incest, doesn't mean we should legislate either way.
I note that you haven't addressed my 4 points about the actual genetic problems, which would actually be relevant to the idea of protecting people. Do you aggree or disagree with those points? If you show me the harm being done, or why we shouldn't apply the same standard to women with X chromosome problems, I'm actually quite easy to convince.
You only offered a vague thing about morals evolving in 6000 years. Well, some did, some didn't, and not always for the better. E.g., we still think that "thou shalt nor murder" is a good idea. (Even if you're an atheist.)
Or conversely the same 6000 years and in the same general region (meaning not just Egypt itself) you move from essentially "believe in whatever deity you will" of the old Egyptians to fundamentalists who'll try to kill you for apostasy or just for being the wrong islamic sect. Or the same 6000 years move you from a culture where Egyptian women were actually rather free by ancient standards, to one where extremists will want to kill one for being raped, or for not wearing a burqa, or for having a job of her own instead of being a man's slave. The same So just because precept X happened 6000 years after precept Y, doesn't mean X is automatically better than Y.
I'd say the same applies to the topic at hand, or indeed any topic whatsoever. Give me a good reason why our morals are better, not just the claim that if it's 6000 years later it must be better.