I Am Against Gay Marriage [Gay Marriage]

Throughout all of history no society ever accepted marriage to be between two people of the same gender. There were plenty of societies that practiced polygamy and polyandry or a combination of the two, but nowhere was the union between two people of the same gender considered a marriage.
And throughout nearly all of western society from the time of Greece and Rome marriage has been defined as the civil union between one man and one woman. Practices such as bigamy and polygamy were outlawed and no polygamous unions were ever legally recognized as marriages, just as no union between two people of the same gender was ever recognized as a marriage.
Before the legalization of gay marriage, marriage for hundreds of years had meant a union between one man and one woman. A problem with gay marriage is that it arbitrarily changed the definition of marriage to include a union between two people of the same gender. Other changes to the previous definition of marriage were not made. Why that particular one?
It seems that if we change the definition of marriage to be between two persons of the same gender, we should also change it to allow for polygamy. Polygamy was allowed in different cultures in the past and is still practiced by some people to this day. Why, therefore, is polygamy outlawed? There are plenty of people who desire to practice it yet they are not legally allowed to do so. Also, why shouldn't marriage be legally recognized between three or more people regardless of their gender? Why is marriage between humans and animals not allowed? Why should two people of the same gender who have sexual relations with one another be allowed to marry, when a man who has sexual relations with two different women can't marry both of them?
It makes no sense to change the definition of marriage to include gay marriage and not any other type of marriage, so this is one reason why I am opposed to gay marriage.