Okay - I just did this once but my computer freaked, so I'm going to try again...
As for cousins marrying one another - this practice is common throughout the world - and I know it is probably a lot more common here in the US than people think. In many cultures, cross cousins (cousins whose related parents are of the opposite sex - my mom's brother's children are my cross cousins and my dad's sister's children are my cross cousins) are the preferred mate - this is due to the idea of lineage (ie you are only related to people through either the maternal or paternal line) which makes the cross cousins unrelated individuals - though through civil practices of land inheritance, etc, they may be ideal mates to increase a young couples wealth (at least, this may be the original reasons, but have evolved over time in the socieities). These cross cousin marriages are common ideas in many places (India, China, Russia and more). Indeed, cousin marriages in general were common ideas into the beginning of the 20th century (when Eugenics began to become popular and some proponents used these flawed ideas to make kissing cousins taboo). I would guess most people have more than two kissing cousins in the family tree if they just looked a few generations back (I know my tree crosses a few times).
Here are some modern websites on romantic cousins:
http://cousincouples.com/http://www.cuddleinternational.org/As for genetics, first cousin marriages (sharing a maximum possible of 1/8th of there genes - while most will probably share less) have a 4-5% risk of offspring with a recessive disease - but then the chance for a couple chosen at random from the population is 2-3.5% so this is not a big increase. Certainly, caring cousin-parents would seek genetic counseling - but then, so should people with any genetic diseases (multiple sclerosis, sickle cell anemia, trisomy 21, etc) in their family history because this helps parents in the decision making process - indeed in the modern world, it is possible for parents to use in vitro fertilization and then preselect the embryo that does not carry the recessive trait (obviously, this is a whole different debate there).
The greatest genetic concern involves the closest relatives (uncle-niece, parent-child, double first cousins). And in all but perhaps the double first cousins (cousins who are children of two related cousin marriages), I would wager there is some serious abuse/manipulation concerns. Anyone who told me they were about to marry someone who was their caretaker or held any sort of dominance/leading role (including even teacher-student relationships) would make me raise an eyebrow. And considering the serious risk of incestious relationships causing genetic diseases/problems (look up "consanguinity risks" on yahoo for some real stats) I can see why the state might have a legitimate basis for excluding relationships between first order (parent-child, siblings) marriages and even second order marriages (uncle-niece, double first cousins) on a public health stance.
But, in general, I think along the lines of Orat. I personally think there has been a huge problem of people thinking the government moderates marriage. It does not, nor has it ever (as evidenced by the polygamous clans). The government moderates civil unions - which are essentially property contracts. Its just that no one has really had to think too seriously in the past whether or not a civil union and a marriage are two separate entities. People enter into marriages for spiritual and maybe moral reasons - to spiritually join with another person, maybe produce/adopt/raise children out of that love, and to express that love as they see fit (short of obviously wrong things like "I hit her b/c I love her"). Therefore, marriage falls into the spiritual/religious realm.
What does fall into the realm of our government is the civil union - the property contract essentially (obviously there are other concerns like health insurance, but I'm just lumping everything under "property" for ease of discussion). I'm sure there are enough marriages that occur for solely this purpose - the furthering of one's wealth in some way or another or protection of one's wealth. So, perhaps there would be a number of civil unions without marriage - and without want for the marriage. I think US Government officials need to stop thinking of themselves as stewards of this great state of marriage and realize they have no affect on the quality of marriage in America. I am currently single and will probably be for many many years to come - but I live with a very good friend of mine and she is also single (and planning to be for many years). We are not homosexual, and most definitely like those of the opposite sex - but as the years pass, we could choose to enter into a civil union - sort of like I would adopt her as a sister or something but express it in a more material way. As we grew into old age, and perhaps neither one of us had any family we communicated with, but one of us grew ill - having a civil union could be a good way for the medical community to legally listen to what my goodfriend/longtime roommate (and now civil partner) says about my medical wishes. Obviously, there are ways for people to get medical power of attorney's now, but few think of it until it is too late. Also, if I died unexpectedly but did not leave a will, or had relatives who wanted to contest the will, as far as dispensation of my half of a co-owned house, a civil union would allow for transfer of the house immediately to my civil partner without great difficulty.
I know many people would say, but thats cheating the government, or if you want to do that you can do many other things to get past the problems - but a civil union (allowable between consenting adults) would be an easy way for people to pretty much cover everything in one quick move. Of course, there need to be restrictions on who can enter into a civil union as far as the intent - like people entering in the union have to live together so many days out of the year unless separated by unavoidable circumstances like sickness, out of town jobs, etc. And you can only enter into one civil union at a time (you have to be released from a previous civil union before entering into another one). This would avoid at least some of the illegal uses of the civil union. Also, breaking the civil union would involve the same divorce process (but obviously, civil divorce would be something separate from spiritual divorce).
And I wouldn't care if multiple people entered into ONE civil union (group marriage, polygamy, polyandry) where all property was combined - so everyone was trully bound to the other. In fact, reading some of the sites about the problems with the Kingston polygamists, etc out in Utah - it seems some of the biggest problems come from the fact there is no recognized civil union with the 2nd, 3rd, etc wife so her children are considered "fatherless" by the state and the women and their children rather illegally get welfare because the husbands income is not counted. However, if these were all legal civil unions, the husband's income would be counted, the children would not be fatherless, and the women would more than likely lose the benefits (of course, the tax evasion by the Kingston clan, and other more devious clans is another issue unrelated to issue of marriage).
So, basically, I think civil unions should be allowed between any consenting adults. There obviously would need to be language written to regulate dissolution of some of the more complex civil unions that could arise (such as who gets custody of the children when one of the parents leaves a group marriage, etc). When I say adults, I mean 18 and older. I realize there are differences in maturity between people - but it seems to me 18 is a good age to set as a legal age of consent b/c in all honesty, I don't think anyone (period!) can ever become totally mature until they get out of high school

! and I was always considered one of the more mature kids who no one would have blinked if I'd married someone at 16. But I know it is easy for someone to act mature intellectually but be very emotionally immature (and for many, never mature emotionally)... I think 18 works as an age of consent because it is impossible to think we should look at the merits of every single individual case and assess the maturity of each individual - these leaves far too much room for human error and even abuse! Plus, those deemed "immature" at 16 would cry out, why aren't we testing my 25 year old friend who acts far more immature than me?
I also think its sick that so many states (my own included!) allow marriage with consent for kids as young as 13. If we are even to allow this marriage with consent, then the minor in question should at least be able to drive (16)! I actually worked with a girl who became pregnant at 12 and her parents forced her to marry the guy when she was 13 - the guy was 17. By the time she was a senior in high school at 18, she had three children with the fourth on the way. All of the children were constantly infected with lice, had terrible hygiene and the mother and father were always fighting and dating outside of their marriage. Of course, the parents thought marriage would solve the problems - but from what I saw it only made matters worse. Interestingly the parents had a similar experience because the grandmother was only 27 years old when the girl got married. So, anyone who seems concerned about the sanctity of marriage, blah blah blah - it seems they should be more outraged by marriages involving minors, marriages forced by parents, and marriages only occurring because a couple have produced offspring!
wow, that was a lot longer than I originally intended