Hi. This is my first post here. First, I'd like to talk briefly on my background. I have been born and lived most of my life in Korea. This country is by constitution and with demographics(over 50% non-religious) a secular country, but bigotry is not unknown here.
Korean laws still have a law against sodomy(not imposed though) and people are quite against gay marriage. (actually, homosexuality is not a big issue here since gay population is quite small.) I think I have seen from the Atheist Experience that there is no good reason against homosexuality except for a religious reason. But I think it's different.
I see secular people here against gay marriage and their justifications for it is not drastically different from what religious folks say. "It destroys traditional values." - is the most prevailing argument known here. I guess it is somewhat along the line of religious objection. I think religion is not the single greatest reason for most people against gay marriage, rather i see it as intolerant people using religion to justify their bigotry. And although from traditional value argument is quite pathetic attempt to condemn homosexuality, I have indeed seen some atheists who are against homosexuality. I just wanted to point this out.
also, I was watching this youtube clip (http://www.youtube.com/watch?v=DPE7orNLt4M&feature=related) and although it is made by christian homophobic group, it had some good points. The clip goes on talking to people who they think are allowed to be married to. And for those people answered with 'anyone who loves each other' the interviewer makes another question. "should siblings be allowed to be married?" and the answer was "no". hence the clip justifies that some things could be plain wrong without good justification. in the end, the last question really got me thinking. Personally, without hesitation, I can say "yes" to the question. As long as they are not having kids(or kids with gene modification to make sure their offspring does not suffer). I do not have any problem with that(let alone why would anyone want to do so??!). But for the people on the clip, there was a moral conflict with their instinctive morals and personal moral code. I wonder homophobia could stem from the same evolutionary moral instinct which made people object to sibling marriage.
If there are no good justifications for prohibiting sibling marriage with no offspring, should we show support on the issue towards siblings-right-to-get-married movement once we cope with the issue of gay marriage? or if not, where should we draw a line? If something is not justified, should it never be imposed with a law? How do other atheists think on sibling marriage?
It's an open question so I'd like to hear other atheists' opinion.
I think the the government can claim a valid secular reason for not allowing siblings to marry if they have the potential to create children (for the genetic reasons you mention). Currently, marriage between first cousins is prohibited for this reason in many jurisdictions.
If you suppose that one of the partners was known to be infertile, that would nullify the argument. Siblings already have many of the rights that married couples do. I can't imagine that if sibling marriages were made legal that many people would want them.
I don't consider this a very strong argument against same-sex marriage.