I guess the Libertarian in me is coming out - it usually does as the weekend approaches! But I am all for the government not getting all caught up in who marries who and who has civil unions and all that jazz.
Hey, do you want to marry your cousin? Go ahead! Why is that my business? Don't give me the birth defects thing, you can be from whole different continents and have a kid with a birth defect.
Do you want to marry Dolly the sheep? Have at it. I really don't even care if they have "relations" with the sheep. They need to watch the movie "ZOO" first (if you have a weak stomach here is the deal: guy wants to have sex with a male horse, with the HORSE taking on the traditionally male role, and ends up with his guts shredded and he died).
How about 6 or 7 wives? Go ahead, marry a whole harem. BC/BS does not have to cover them all. BC/BS is a private entity and they can say "we only cover one spouse". Have fun dividing your life insurance between a bunch of women. I'm not sure what other benefits people are wigging out about.
If two adults of either gender want to get married, have a civil union, adopt a kid, go to church, eat cookies in bed, whatever - have at it. I don't feel like it is a negative thing for society. Not too long ago society was shunning divorced people, people who got pregnant out of wedlock, and people of different races who married. I think as society evolves these type of laws that impose one groups morals and standards on other groups will disappear. When anyone's rights are restricted, all of our rights are restricted.