1. Denying them is a violation of religious freedom (civil and religious marriages are two separate institutions). The main reason for denying marriage to gay couples is that all major religions consider homosexuality a sin; however, the First Amendment of the Constitution clearly states that a person's religious views or lack thereof must be protected. Marriage by the state is a secular activity; the government cannot start making laws just because a religion says they should. What's next, should we make taking the Lord's name in vain a criminal activity because Christians consider it a breaking of a commandment?
2. Marriage benefits (such as joint ownership, medical decision-making capacity) should be available to all couples. Marriage is more than a legal status. It affects many things in society such as tax filing status, joint ownership of property, insurance benefits, and agency law. It affects critical medical decisions. For example, if one member of a gay couple that has been together for 20 years gets critically ill, visitation may not even be allowed since the other isn't considered a "spouse or immediate family member". Also, critical medical decisions must often be made when one person is incapacitated; e.g. should a certain surgery be done or not? It is completely unfair to deny these privileges to people because their relationship doesn't fit the state's definition of one.
3. Homosexuality is an accepted lifestyle nowadays with most evidence strongly supporting biological causation. For too long homosexuality has been considered a form of "deviant sexual behavior". Those making these accusations should examine the history books and the psychological research. Throughout our history going all the way back to ancient Greece, homosexual relationships have existed. The term "lesbian" comes from a Greek island called "Lesbos" where many such couples lived. An overwhelming amount of research has been done showing that