In some ways I am against gay marriage, just like in same ways I am against hetero marriage. The reason I am against gay marriage in a way is are some gays getting married to actually get married and make a commitment to each other or just trying to rebel against the establishment? If the marriage is on the premise as a word to provide protection between both assets for financial, medical, and family matters than yes, gays lesbians, heteros, trangenders, etc. should be allowed to be married.
If we define marriage as a civil right (which we do, since marriage licenses are issued by the government), you legally cannot discriminate among citizens when issuing licenses. Section 1 of the Fourteenth Amendment and the subsequent incorporation of that amendment (its application to federal, and not just state, laws) require just that:
"No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws."
Note: The latter half of this argument applies only, of course, to Americans.
If you look up many cases, children raised by gay couples tend to turn out heterosexual so the upbringing of the child does not show gay indoctrination and the lack of marriage between a homosexual couple brings more harm to the children of homosexual couples due to the lack of legal protections the family has if something were to happen.