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Honestly, I don't really know how to approach this, especially since so many psychological issues are at hand here.

Aside from societal conditioning, i.e. the "icky factor", there are genetic and psychological explanations for the incest taboo.

The genetic arguments against are basic high school biology stuff. A pairing of individuals sharing alleles is more likely result in homozygous recessive offspring, which would display serious weaknesses. One expert has explained a situation which would demonstrate long-term benefits, but it's impractical due to the short-term implications

The psychology, which suggests that animals recognize the genetic effects of inbreeding, shows that some species possess an evolved aversion to inbreeding based simply on kin-recognition.

In humans, there is the Westermarck Effect, in which children who are raised together from an early age, blood related or not, have little sexual attraction to each other. However, when siblings are raised apart and meet outside of the years which result in the Westermarck Effect, they are very often attracted to each other. This is due to genetic sexual attraction, in which people are predisposed to look for traits similar to their own in a potential mate.

But that's just science. My personal opinions?
Where incest crosses over with child abuse, it's still child abuse and therefore wrong, even more wrong, in fact.
Adult incest is iffy. Keeping in mind that the term incest refers only to sexual activity, not the production of offspring, which is inbreeding, I wouldn't call adult incest wrong, but I still don't approve of parent child incest, even as adults. As far as incest with cousins, and please remember that I mean sexual activity, not full blown inbreeding, I don't have a problem with it.

The answer to that question largely lies in the cultural and personal values of the person at hand. A good chunk of the time you'll get the standard 'Sick ********' response, even in some of the more open minded cultures in this world.

Technically, incest wouldn't produce birth defects or other problems if we could turn off genes that are bad. The reason we get birth defects is because our X and Y chromosomes are full of junk. Stuff like "tail" genes and "gills" and the like. If you have two people who are related, they're more likely to have two chromosomes that are the same, so the defect shows up. The way your X and Y chromosomes work is like when you're using bit-torrent, only there's two torrents. You have one finished product (baby) that you're making, but you're using bits of two different sources to create it. If both sources has a bad spot (like Down Syndrome), than the baby will probably be born with it.

This is another reason why males tend to have more genetic defects than females. The Y chromosome is TINY compared to the X chromosome, so if there is something "bad" on the X, there is nothing on the Y to use to make up for it, so that's why males tend to be more likely to get things like color blindness.

So, technically, if we could perfect genetic manipulation and "delete" the bad crap off our genes, incest sex could technically create babies with no defects whatsoever.

I don't think incest is in the same taboo zone as homosexuality, at least not exclusively. I don't give a damn either way. If the catholics are right, we're all going to hell, albeit for different reasons.

Now, don't let that fool you, it's just that incest isn't something I feel strongly about. Sure, there are serious genetic risks in inbreeding, but then people would argument about not having children, rendering my argument useless. I've learned not to care about what other people do to or with their genitalia.

Oh my...Well, all of the following are personal opinions of mine that have been based of my interpretations of society and morals into my own personal life.

I feel that incest between fully (not half) related brothers and sisters is wrong. That genetic make-up (plus or minus a y chromosone) could just as easily have been yourself, and the connection is so close that the idea of intimacy to the point of procreations seems unnatural (i.e. hermaphrodyte having sex with themselves)

Now, as for half siblings, there's less genetic make-up, but becasue of the average situations of parents divorcing and remarrying to start a new family, half siblings tend to grow up with each other so sexual feeling will probably not arise (i.e. 7 year old's mom remarries and has another kid, a half sibling. They grow up together like full siblings)

As for step siblings, I see no problem at all. If they were not raised under the same household, perhaps meeting at 14 and finding feelings for each other at 16, feelings that arouse are natural, especially if the other falls under their definitions of "beauty". Since their genetic make-up is NOT from teh same donor, they have the same chances as if their parents had not married and they became siblings. I only bring up the steps because no one else has and society frowns upon this as well, even if it's less stern.

When it comes to siblings seperated at birth and find love 20 years later (i.e. maury show) it just sucks for them. Society will judge them, but they did not have the opporunity to grow, and are strangers, if not genetically. I do not havea problem with their relationship unless they decide to procreate. Becuase of the high risk of birth defects and social problems their offspring will face, I'd rather that not happen to a child.

Cousins aren't as closely related, and I've never had an attractive cousin nor one of similar age, I really can't say much about this. Yes, there's a higher chance of birth defects, but less so that you just have to think of it this way, "At least they aren't brother and sister". I think this estranged brother and sister argument can be used for this as well. Cousins who never met until teen years or later and who develop sexual attraction will face hell from society, but as long as they think of the troubled kid they might produce, it's their choice.

As for your dad's cousin's kid, I don't give a ********. Their relation is so far off, it doesn't matter. They might as well be strangers and I say go for it if you can get it. If you're worried about family feuds, then either leave for hawaii or tell them to ******** off.

So, Summary?

Brother/Sister=No Go Zone
Half Brother/Sister= Danger Zone
Step Brother/Sister= Safety Zone
Estranged Brother/Sister= Serious Danger Zone(Circumstances)
Cousins=Danger Zone
Far Off Cousins= Safety Zone

All in all, we have our own lives to lead and consequences to suffer. If My brother and sister got married, it'd be werid , REALLY weird. But, when it comes down to it, I love them both and would have to respect their decisions and warn against all dangers.