[QUOTE=DianaG]
I’m not. Speaking as a former child, my father is not the boss of me anymore. Yes, he was the boss of me for eighteen years, but that was half a lifetime ago, and we’re pretty much over it. Speaking as a parent, I expect to be not the boss of my daughter at some point in the near future. We will, at some point, relate as equals.
Again, I’m not saying that I think that’s *always * the case (because of course I think that there are so many power issues involved that I can’t even conceive of, let alone anticipate all of them), and I literally could not be more freaked out by the idea of incest in general, but I think that, for the most part, it’s not my place to tell consenting adults what they can and can’t do with their bodies.
[/QUOTE]
I think the stumbling block for a lot of people (and I know it’s part of my own) is that the parent/child relationship is *by definition * one of power imbalance for at least part of its duration. Parents do have authority over their minor children - for a goodly chunk of childhood they have essentially absolute authority over them. That period of absolute authority embraces pretty much the entirety of one’s formative years. The part of life when you learn what is and is not acceptable and appropriate behavior.
My father hasn’t been in a position of authority with me for well over a decade - I’m an adult, have been legally one for 15 years and functionally one (as in, no longer beholden to my parents for financial support) for 11 years (since my graduation from my bachelor’s program). We’ve gotten (after some work) to the point where we’re friends as well as father and daughter, but if he tells me a behavoir is or is not appropriate, I’ll give his opinion vastly more weight than I’d give the opinion of some guy trying to pick me up in a bar.* His opinion of what is and isn’t correct behavior is what shaped my own sense of right and wrong in large part. I’ve got a lifetime pattern of respecting his viewpoint on such matters - I don’t always agree now that I’m an adult,** but I’m still more inclined to trust his judgment than average.
Generally, a person’s parents enjoy a special level of trust. Parent-child incest just absolutely reeks of abuse of that trust. It also warps the hell out of the other family bonds just by it’s nature. How can it not warp the bond (taking the case in question as our model) between the daughter/wife and her mother? Between herself and her other siblings? Between the father/husband and* his * other children? If your dad is fucking your sister (even if she was fully adult when the relationship moved to that phase), how does that not affect your relationship with the both of them in a bad, bad way?
There comes a certain point when one is piling constraints and limitations and conditionals on a certain behavior that a flat prohibition just becomes the sensible and reasonable course of action. I’m having to fight off the feeling that some of the people advocating we just live and let live for all sexual behaviors (because to do otherwise is just to be irrationally prejudicial) are just freaking rules-lawyering for the sheer fun of it. Not all social stigma are arbitrary and capricious - they’re not all irrational. Granted, some of them are, but not all of them. The incest taboo isn’t an irrational one - for a whole host of reasons. Even the incest defenders in the threads have (generally) essentially conceded that - they’re just arguing that in this case because of X and Y and Z seriously improbable circumstance we should just leave these people alone with their taboo-breaking kink.
*Assuming for puposes of this discussion I weren’t happily married and therefore immune to getting picked up by guys in bars 
**For example, he and I disagree on various political issues and he’s got a prejudice against a certain ethnic minority I do not share.