Greentext

And if people were killing their viable births why would they fret over the non-viable ones?

That's exactly what I am saying. I put the wrong word in the last comment.

That's the part you think I made up?

I'll admit I can't source that one, most of the studies are on cousin fucking but it would reason the most genetically diverse a couple is the less common abnormalities.

First off that's not how that works. If two individuals are two far apart genetically they can't breed at all or they would have disabled or infertile offspring. That's why you can't mix random species.

Second lookup how human mating selection works. There are mechanisms in place to ensure people with the right genetic compatibility are the ones breeding. Part of this is actually making sure they are similar enough to be viable, while still having some things like the immune system be distinct. This deals with inbreeding and makes the population as a whole less susceptible to a single infectious agent. Sure this is far from bulletproof but without extensive genetic testing you're not really going to be able to do any better. There is no easy way to tell what your genetic compatibility is with someone.

There is no reason to assume they aren't compatible from a single miscarriage. Not all miscarriages are caused by genetic issues. Thanks to things like de novo mutations and cross-over each embryo has unique genetics anyway, so genetic issues in any given symbiote don't mean the couple which produced them has issues with genetic incompatibility in general. It could simply be bad luck, or many other factors like the age of the couple or exposure to radiation or environmental toxins all of which cause genetic and fertility problems.