What if pigs are better at growing human fetuses than human wombs... an interesting concept. I guess this depends on how you define "better." I guess if you look at a human as nothing more than a collection of cells that are either more or less robustly healthy, than it might be possible to say that a pig can grow a human faster, or larger, more reliably, or with softer hair, or nicer skin tone, etc than a human. But ultimately, we just don't know what might be lost by having a non-human mother.
We have to live in the same world as these hypothetical humans - how could be we ever be sure that these people were not damaged fundamentally by being created in a non-human environment?
We are finding out now that maternal stress levels in pregnancy can change the brains of developing infants, to emphasis strength and physical attributes over intellectual attributes. If something as simple as stress can make a huge difference in the person we become, it seems incredibly risky to grow a human in an non-human environment.
I'm not arguing that the law has merit insofar as the law is supposed to be reactive and not proactive, but surely we can acknowledge that the limited and evolving nature of science is actually a strong argument against this kind of thing. These humans might well live to see the science that created them be discredited.