Should Wendy and Carol be allowed to legally wed?

Should same sex marriage be legal? I think so. Marriage is defined as the union between a man and a woman. Why? Why does it have to be a man and woman? Where did the idea that marriage is union of a man and woman come from? The Bible? I don't agree with that principle. Prayer was taken out of public schools to seperate church and state, so why are we holding on to the idea of marriage as being a man and a woman? Personally, I don't agree with same sex marriage. I definitely like the benefits of the opposite sex. But I don't have any problem with someone does like the same sex as them. I think if two people love each other and are willing to make that commitment, then they should be allowed to legally marry. It doesn't have to be a religious ceremony. Civil ceremonies are done all the time.
So, what's your opinion? Is this something that should be changed? What are your personal beliefs? Thanks in advance for the input. All comments are appreciated. :)