Marriage is defined as:
- The legal union of a man and woman as husband and wife.
- The state of being married; wedlock.
- A common-law marriage.
- A union between two persons having the customary but usually not the legal force of marriage: a same-sex marriage.
The last definition is highly controversial, as gay marriage is a hot topic in US politics.
Christians claim that marriage is strictly a religious tradition, and that government should not be involved in marriage at all.
In fact, marriage only became religious in nature thanks to a concerted effort by the church. People have been getting "married" for most of mankind's history. The act of getting married deals with far more than religious ceremony; it involves family, jealousy, and questions of paternity.
In the 16th century, the Catholic church passed the Tametsi decree, which declared among other things that no marriage was legitimate unless recognized by the church. Thus, the church declared themselves effectively in charge of families by fiat.
The claims many theists have that marriage is a religious matter and that government should stay out of it could be correct. However, governments are often involved--as is the case in the United States--in what has been called marriage due to medical decisions, tax filing among many other things afforded to couples who are married. Since we live in a society that uses marriage in these ways, rather than just two people who are "married" in a religious ritual, anyone should be able to get married legally in order to be given the same rights, same-sex or not.