Don't have sex with your siblings
[...]Over the next few years we had sexual encounters every six months or so, each time going farther and farther until I was 17, when we had full sex for the first time. We both went out with other people and there was never any jealousy, although I found it hard to be physically intimate with anyone else. Part of that was because sex with Daniel was so amazing that I had no patience for all the fumbling that seemed to happen with other boys. The sex was never pre-planned, but just always seemed to happen when there was no chance of being discovered.(HT: Rod Dreher via Ross Douthat) The title of the essay is "I had sex with my brother but I don't feel guilty". Rod Dreher asks:
If God doesn't exist (that is, if there is no such thing as absolute moral truth), why shouldn't the woman have sex with her brother? They're careful not to risk reproduction, its always been consensual, they enjoy it, and they don't feel guilty. So what's the problem?Well, Mr. Dreher, here's my answer to that, as I posted in the comments to both your blog and Ross Douthat's:
Here's a practical argument for maintenance of this part of the incest taboo: It's a bad idea to have sex or romantic relations with your siblings because you can't dump them when things go sour. The vast majority of sexual and romantic relationships do not end with blissful matrimony. Often, the end is accompanied by very negative feelings. When this happens between unrelated people, the negative feelings can be ameliorated by the former partners distancing themselves from each other.
With siblings, such distancing is extremely difficult without inflicting damage on the whole family. They're your sibling no matter what. You can't dump your brother into un-brotherness. And it puts the rest of the family in an extremely difficult situation, much as a divorce does: who do they "side" with? If the formerly incestuous couple can't be together at family functions, who gets to be there? etc.
Another reason to avoid becoming entangled with your siblings in this way is that even if somehow you do manage to make the relationship "work", children would be at enormously increased risk of birth defects. Often, you want to have babies with the person you're in love with. This drive can be extremely powerful. Having that person be your sibling makes that drive very perilous to succumb to.
So one needn't resort to esoteric theological arguments to find a justification for the incest taboo. It has enormous practical benefits that become obvious if one thinks ahead to the likely consequences of the incest.