system
Given the following post, respond as if you're commenting on Reddit:
human
I don't think that incest between siblings should be illegal. CMV.

This thought occurred to me when the governor of Virginia recently compared gay marriage to marriage between siblings, and everybody freaked out. Because once I got past the societally-ingrained ick-factor (just like gay marriage), I couldn't think of a good reason why sibling-marriage would be such a bad thing that we would keep it illegal (remember that there are lots of harmful things people can do that are still legal---it has to infringe on somebody else's rights or be particularly destabilizing for illegalization to be justified).

Basically, from an anthropological perspective, the way I see it is, humans figured out that bad genetic stuff happens when relatives produce offspring, so it became a big taboo. But these days, we have birth control, genetic counseling, and there's very soon going to be a lot more genetic tools at our disposal to make sure that that kind of thing doesn't happen. 

I myself don't have a sibling, or even a cousin, of the opposite sex, and have never been remotely attracted to anyone in my family, so I don't have a dog in this fight. But I know from perusing places like literotica.com, where like all the top rated stories seem to be incest stories, that familial attraction is something that a lot of humans go through. And I imagine that a brother and sister who grew up together and already love each other as family, if they got along and were compatible in other ways, might make really good life partners.

So, given the new genetic tools that we will have in the next several years, why do you think that marriage between siblings would be so damaging to society that it should remain illegal?

(This is a pretty controversial view so I'm using a throwaway.)
gpt
The problem with sibling incest is that it is too often the result of abuse.  There may be nothing wrong with two people who are genetic siblings getting together if they just met as adults, but this is by far the least likely case of incest.  Sibling incest usually starts young, when the age difference between the siblings comes with a large power disparity.  This makes the chances of having a non-abusive relationship between siblings fairly low and can explain modern laws against sibling incest.