There are many things that parts of the Bible say are wrong but they are legal in our society.. gluttony, greed, pride, gambling, etc. The Bible is a moral guide, not a legal one.
The notion of marriage as a sacrament and not just a contract can be traced St. Paul who compared the relationship of a husband and wife to that of Christ and his church (Eph. v, 23-32). (From a history of marriage)
http://marriage.about.com/cs/general...agehistory.htm
Yet, marriage existed long before that. So, it is ok for current Christians to protest changing the definition of marriage yet it was Christians who changed it then? It has changed many times over many cultures over many years.
Heterosexuals can get married without including religion in any way. Any legal references to marriage are only legal, they are not religious. In the eyes of the law you are just married whether you are married by a Justice of the Peace, a Catholic Priest or a Rabbi. Therefore, any religious arguments against gay marriage are illogical because there is no religious component to our current marriage laws. You can choose to be married by a religious officiator but it certainly isn't a requirement.
As to whether homosexuality is a choice, I truly find it hard to believe that you can control who is sexually attractive to you. There are men who are only attracted to blonde women or only attracted to red heads. There are women who are only attracted to tall men or men with facial hair. You can't even control who you are sexually attracted if you are heterosexual! Many of us have met someone of the opposite sex who is kind, fun, and a good friend, but we just can't think of them "that way" for some reason. Sometimes we try very hard to, because we hold that person in high regard and think that we *should* be attracted to them, but we still are not, no matter what we do. You cannot force yourself to be attracted to someone who is not attractive to you. Why would this be any different for homosexuals? If you are not attracted to members of the opposite sex and the only people you have ever been attracted to are of the same sex, then what are you supposed to do?
I never will be able to understand why anybody is against gay marriage as a legal institution. It isn't as though anybody who is going to force someone into a gay marriage against their will. If consenting adults want to pledge themselves to each other for a lifetime, then why shouldn't they be allowed to?