Nope.
I could bring up Bonobos, that screw everyone around them regardless of age, gender, or relation, but that wouldn't really argue my point.
One can't merely use "herp derp genetic defects" as an argument, since that doesn't say anything about homosexual incestuous relations or those with a partner that is sterile, and also fails to take into account that even a brother-sister relationship with children has only a 1% higher chance of mental defect in the kids when it's the first case of incest in the family (that is, the parents of the brother-sister aren't related), making it more negligible than a person with a big family history of cancer having kids, or, in the case of my family, a lethal genetic defect that has affected (killed) 3 of my 9 great aunts/uncles on my mom's side of the family, and also cystic fibrosis, which my 2nd cousin's twin daughters have. On top of that, my great grandma had (but survived) breast cancer twice, and my OTHER 2nd cousin has Hodgkin's lymphoma, which is basically a cancer of the blood. That's all on my mom's side. On my dad's side, my grandma died of Lupis (I know I know, it's never Lupis).
...Yeah. I have a far higher chance of at LEAST passing on two or three LETHAL recessive defective genes to my kids than the average person and his/her sister/brother. Interestingly, though, asthma affects incestuously-born children far higher than normal children, and at a rate far higher than the 1% for physical/mental defect.
There is no other reason to reasonably say that incestuous relations are bad except for, in the case of parent/child, the position-of-authority issue. The "unnatural" argument doesn't fly since a great many types of relations can be viewed as "unnatural" based solely on its prevalence in ancient human or animal populations.