Theey are not based on reason or facts, they are based on careful interpretation of a religious text to fit your chosen point of view.
Even though the Buddhist texts are silent on the subject of monogamy or polygamy, the Buddhist laity is advised to limit themselves to one wife.
http://www.budsas.org/ebud/wha...
By getting married you are not just getting a wife, you are getting your whole world.
http://www.jannah.org/sisters/...
In traditional Judaism, marriage is viewed as a contractual bond commanded by God in which a man and a woman come together to create a relationship in which God is directly involved
http://en.wikipedia.org/wiki/J...
A custom among the Northern Californian Native Americans*, which was unique to them, is that of half-marriage and full-marriage.In a full marriage, two kinsmen represented the future bridegroom. After agreeing on a price, in accordance with the family’s wealth and social standing, the bridegroom – usually with his father’s help – would pay the bride’s family.
http://www.weddingdetails.com/...
It appears, that your own bigotry is showing. Most societies marriage was between man and woman. In many of the others plural marriage was also allowed, but it was almost exclusively Heterosexual.
I put "gay" in quotes, because the original meaning "happy" no longer applies. And having seen my brother's friends (he was homosexual) "gay" meaning happy was laughable. Most of them were miserable sorts.
Do you not understand that there are very sound and well established, science based medical reasons why a union between close family members should be banned?
Is marriage about sex, or status in society or benefits granted by governments. The moment you decide on one, let me know. Because you don't need "marriage" for any or all of those. ;) But logic be damned we want to re-define "marriage" so that we can have our sex, status and most importantly benefits!