1) Some people get 
abused by relatives. That makes 
consensual incest wrong the same way 
abuse/assault by non-relatives makes sex in general wrong.
2) Many people are disgusted by the idea.
 If something disgusts many people, it must be wrong for everybody!
 People are so disgusted by the possibility of having sex with a close
 relative that they always get DNA tests before having sex with anyone, 
to just be sure they aren’t having sex with a relative.
3) 
It increases the risks of birth defects,
 and anything that does that is wrong and shouldn’t be allowed, just 
like we don’t allow pre-menopausal women over the age of 35 to have sex,
 and we don’t let anybody with obvious, serious inheritable diseases 
have sex. Yup, this is why it is wrong for two half-brothers to fall in 
love, or why stepsiblings who didn’t even meet until they were teenagers
 shouldn’t be together. They might make a mutant baby!
4) It’s 
illegal in some places,
 and something being illegal always makes it wrong. You know, like 
harboring runaway slaves? That’s why having sex with your first cousin 
is wrong in Texas but just fine in almost every other US state, half of 
which legally marry first cousins, and why consensual incest between 
closer family members isn’t wrong in Rhode Island, which has no laws 
against consensual adult incest.
5) It’s not natural, and people should only be allowed natural things, like bicycles, smart phones, and iPads. OK, maybe 
it is natural in some species. But we shouldn’t lower ourselves to the behavior of other animals, who make wars and pollute the planet.
6) 
Someone’s religion is against it. And if someone’s religion is against it, 
nobody else should be able to do it. You’ll never find 
examples of acceptable consensual incest in the Bible.
7) There are so many people you’re not closely related to. 
That makes consensual incest (consanguinamory) wrong, just like there being plenty of people in your own race makes interracial relationships wrong.
8) 
Only rural poor people would ever do such a thing, not 
royals or 
educated people. And anything done by rural poor people is wrong.
9)There is often a 
power differential
 in consensual incestuous relationships, and relationships with power 
differentials are just wrong. That’s why no President of the United 
States, Senator, Governor, judge, district attorney, or police chief has
 ever been married, and we bar wealthy or intelligent people from 
marrying someone who isn’t as wealthy or intelligent as them. Yes, power
 differentials are exactly why half siblings close in age, even if they
 didn’t meet until they were adults, shouldn’t be allowed to be together.
10) It messes up 
family structures and dynamics.
 That’s why every family’s dynamics are always required to be evaluated 
and corrected by outsiders, and people are never allowed to break up if a
 breakup will mess up the dynamics of the family. And people are never 
allowed to work with family members, as that could cause conflicts or 
too much reliance on family. Yes, messing up a family dynamic is why 
genetic relatives who were raised by different families should never be allowed to be together.
Yup, 
we need to let 
all of those people
 who’ve found that a close relative makes the best life partner for them, or 
perhaps just a trustworthy sexual partner, know what they are doing is 
wrong
 and they should stop, and go settle for someone else, who I’m sure will
 be just fine being the B-list choice for someone who’d rather be with 
the person they see when the family gets together. People need to make 
sure they aren’t doing anything that makes anyone who’s not involved 
uncomfortable. That needs to come before their happiness.
This bit of sarcasm is brought to you by someone who supports the rights of ALL consenting adults to their relationships with any and all consenting adults.