In Germany, there are brother and sister who are fighting for the right to continue having sex with each other. The story goes that the brother was given up for adoption and didn't meet his sister until he was 18. They soon fell in love. (Strangely, this is pretty common for siblings who meet for the first time.)
After the couple met and began a sexual relationship, they had four kids, with tragic results:
All but one of them have been placed in care and two are mentally damaged as a result of inbreeding. In 2002, Mr Stübing was given a one-year suspended sentence after being found guilty on 16 counts of "illegal coitus" with his sister.
My personal morality stems from the idea of "your right to extend your fist ends at the tip of my nose," or, if you're not hurting anybody, what you do is your own business. So masturbating while thinking of a child sodomizing a sheep might reveal that you have some mental problems, but I wouldn't consider it immoral. Stealing, murdering, abusing, etc. are immoral because they cause harm to others.**
It's clear to me that having kids through incest should be illegal because of the grievous and unnecessary harm to the children. Although this belief has some troubling implications, not least the very slippery slope towards criminalizing having children with people with low IQs, minor genetic disorders, etc., I think that in instances where the harm is grievous and predictable, like brother-sister incest that leads to children, we can safely make that ruling.
What of siblings who have no chance of having children?
Mr Stübing, an unemployed mechanic, was released from jail last year and is still living with his sister. Although he has had himself sterilised to avoid fathering more children, he could be sent back to jail at any time for persistently reoffending.By what justification may the government prevent them from having sex today? That most people find sibling sex repulsive? Repulsion is no basis for law -- we allow neo-Nazis to speak, after all. (Actually, Germany doesn't, but even though I disagree with that stance, there's at least conceivable grievous harm that stems from allowing hate speech.) If religion is a suitable basis for secular law, then the case is clear, but as you might imagine, I don't think religion is a good basis for law. The only other arguments I can imagine are similar to those used against gay couples -- that they set a bad example, or otherwise pose some threat to "traditional" families or children. I do not find such arguments convincing.
I therefore believe that consensual, adult incest should be legal and is not immoral, provided there is no chance for children to result from the union. Where to draw the line on infertility is another troubling and complex question, of course. Simply using condoms or the pill is not sufficient to rule out the possibility of children, but having the government mandate surgical sterilization is distasteful, to say the least. While I'd encourage siblings who get pregnant to have abortions, government-mandated abortions would be deeply disturbing as well.
I think the only coherent policy is that brother-sister incest that may result in children is disallowed as reckless endangerment. The government should not mandate sterilization, but simply recognize that the problem is the reckless endangerment of offspring rather than the act itself.
* For the record, I believe adult, consensual polygamy should be legal, although not necessarily institutionalized by the state as marriage, and am unsure about bestiality because of the question of the animal's consent. Bestiality should be treated as a subcategory of animal cruelty .
** Things get complicated when all choices are harmful and you must pick the least harmful one, like killing in self-defense or putting violent criminals in jail. There are also some gray areas in which people can reasonable agree like whether a aborting a fetus is a significant harm or whether it's okay to harm non-human animals or other organisms. (I eat meat, but even most vegans are happy to eat plants, fungi, and bacteria.)