I totally agree with you, and there is a significant chance that their incest-born baby will be born without any malformations/deformities/genetic disorders.
Yeah, but the chance is just as significant, if not even more so that it WILL have genetic defects. Depending on the study, the averages are between 33%-73%, but the accepted average usually falls around 50/50. The problem is though, that only refers to the first generation of offspring. By the third generation's offspring that average is almost always a sure 100% genetic defect in the offspring, where the only differences in averages are how lethal or sever the defects are.
Well it doesn't really matter anyway, because in most modern, civilized places it IS very illegal, just like drunk driving or shooting someone in the face with a gauge. That will never change. It's not only considered morally and/or social wrong, but medicine and science both are on the side of the advocates against incest.
Sure love is a wonderful thing and it kinda bothers the soul to interfere with it, but I can pretty easily quite my soul, when I think that my ENTIRE damn species is at stake, in the long run. It seems that some people are only thinking about the immediate future or first born, but if it was allowed or made legal, it wouldn't stop with just the first generation, which kinda has a gathering snowball or domino effect.
Pure, simple and this was learned centuries or more ago (mostly by farmers and/or animal breeders) as well as very proven today with studies along with modern research, that inbreeding IS bad, being a direct path to the extinction of the breed or species. Sure it won't happen in the beginning, but it is proven fact that if a species is allowed to breed incestuously, unchecked, eventually the gene pool becomes so damaged that it can no longer maintain or support its species. FUCK love, I'd rather be sure that the future generations of my bloodline have a healthy society to live in. (YES "healthy society" is a VERY relative and mostly inapplicable term

, but I am only referring to incest not being allowed and staying that way in society.)
Even "wild" animals don't inbreed or avoid inbreeding unless they are in closed captivity or in some type of isolated environment, that alone should be kind of a big hint.

LOL!