Further Reading:

Article:
A Reconciliation on Gay Marriage by David Blankenhorn and John Rauch, New York Times, February 22, 2009

Books: A Time to Embrace by William Stacy Johnson
    Excerpt (chart): "Same-Gender Relationships in the Church: Seven Theological Viewpoints"
 The Future of Marriage by David Blankenhorn
 Virtually Normal by Andrew Sullivan
 
Websites:

 Gay Marriage Issue Resources: The Pew Forum on Religion and Public Life
  This issue page from The Pew Forum, a non-partisan, non-advocacy organization
  that takes no position on policy debates, offers an excellent collection of legal, political
  and theological resources about the gay marriage debate.

 Institute for American Values: Resources on Marriage

 Advocates and Advocacy Groups:
  CherylJacques.org
  DOMA watch
  Freedom to Marry
  Family Research Council
  Human Rights Campaign
  Institute for American Values
  Religious Coalition for the Freedom to Marry
  MassEquality

 Official Statements of Religious Organizations:Many of these links were assembled by the Pew Forum on Religion and Public Life (see above).

 Roman Catholic Church:
    Congregation for the Doctrine of the Faith:
      * Statement on Unions between Homosexual Persons (March 2003)  
      * Statement on Pastoral Care for Homosexual Persons (October 1986)
    United States Conference of Catholic Bishops (USCCB)
      * Marriage and Family 
      * How does legalizing same-sex marriage deny the true nature of marriage? 
      * National Pastoral Initiative on Marriage

 Episcopal Church
 Islam 
 Judaism: 
     Reconstructionist
     Reform
     Conservative
     Orthodox
 National Association of Evangelicals
 Southern Baptist Convention
 United Church of Christ
 United Methodist Church

 Other Documents:

Federal Marriage Amendment (Proposed) - Library of Congress
Federal Defense of Marriage Act - signed by President Bill Clinton in 1996
Goodridge v Dept of Public Health - Massachusetts Supreme Judicial Court Decision (2004)