Taboo

Clearly the fight against gay marriage is a fight over what and what is not taboo in our society. Conservatives have already lost this one, whether they like it or not. Homosexuality is barely taboo anymore, and mostly it's not. They like to compare it to the incest taboo, but that argument only makes sense if you think that incest means the same thing in every time in every culture, and it doesn't.
For instance, in the Bible it's clear that in ancient times the incest taboo existed for siblings who shared a mother, but not a father. Abraham clearly states that Sarah is his paternal sister, and at the time that wasn't incest because families were still matrilineal instead of patrilineal. Or in recent times in Europe, marriage to a first cousin was not incest like it is in this country. Einstein and Darwin were both married to first cousins.
The incest taboo is extremely flexible AND extremely powerful. The arguments based in some sort of scientific meaning are just so much bullshit. Incest taboos reflect familial culture more than anything else. The closer your relative is to how you live your life, the more likely it is incest to marry him/her. On top of that, the more people you know, the more relatives are considered off-limits to you. More access to more partners has more to do with why cousin marriage is illegal than any genetic argument.
Taboos are created by societies, not by "nature". Our society does not lend itself to a taboo against homosexuality anymore. That's a leftover taboo from highly gendered societies that don't respect individuality. We're free, and it follows that homosexuals should be free, too.
The flaw in comparing homosexuality and incest is immediately apparent to anyone who is even slightly educated in anthropological findings about taboos. But if you don't know why something is taboo or that taboos change, you are suspectible to arguments comparing one not-really-taboo-anymore behavior with something that is strictly taboo because there's no real way to know the difference.
I see that flaw going on this this discussion. Arguing against homosexuality by drawing on unrelated sexual taboos is strictly an accident of bad education. But it's an extremely common lack of knowledge. How to explain that without sounding like an "elitist liberal"?
One more thought on taboos and the "slippery slope" argument. The argument against homosexuality is that if that ceases to be taboo than the barn door is open and all taboos will become acceptable. Evidence for the validity of this tends to come in the form of "First it was the pill...." type arguments. Intelligent people are running around saying that we have to shut down rights for homosexuals not out of hate for them so much as if we don't have some grossly unfair laws enforcing meaningless taboos then society will go to hell in a handbasket. But even though the law has little bearing on what is taboo, even if it did this argument would make no sense. Taboos change and society soliders on. For instance, people eat shellfish and pork without any noticiable downhill slide towards paganism and anarchy.
So, even if you do accept the argument that minority rights should be withheld to cater to hysterical taboos held by the majority if it will keep chaos at bay, there really isn't a reason to think that the taboo against homosexuality, which is really 75% gone already, is going to make a difference.