Source: Pew Forum report, Los Angeles Times
Just after the California Supreme Court ruled on May 15 that marriage is a fundamental right of all couples, including same-sex couples, the Pew Forum on Religion and Public Life released a revised version of a study it did to assess the positions of various religious denominations on marriage of gay or lesbian couples.

Although the California ruling applies only to civil marriages, the attitudes of churches became significant partly because several conservative congregations supply the political force behind an anti-equality amendment that is expected to appear on the November ballot. If passed, and if it survives expected legal challenges, the ballot measure would once again make same-sex marriage illegal in the state.

The Pew survey found only a few US denominations that give unequivocal support to marriage equality:

• Unitarian Universalist Association of Congregations passed a resolution in 1996 in support of same-sex marriage.
• United Church of Christ General Synod voted in 2005 to legally recognize and advocate in favor of same-sex marriage.
• Judaism -- Both the Reform and Reconstructionist movements support gay and lesbian rights, including the right of same-sex couples to wed. The Conservative movement does not sanctify gay marriage but does grant rabbis the autonomy to choose whether or not to perform same-sex marriage ceremonies.
• Metropolitan Community Church wasn't included in the Pew list, but is sometimes called a "gay church" because it was started to provide an alternative for LGBT people to traditional Protestant denominations.

Other American churches where LGBT members sometimes find what's called a "welcoming and affirming" atmosphere include Episcopal, American Baptist, Presbyterian, and American Lutheran.

Attitudes vary from one congregation to another. (See references at the bottom of this post for lists of individual welcoming congregations.)

The Episcopal Church has not explicitly established a position in favor of gay marriage, but did, in 2006, state its "support of gay and lesbian persons and [opposition to] any state or federal constitutional amendment that prohibits same-sex marriages or unions."

LA Times reports that at least one Episcopal congregation in California, All Saints Episcopal Church in Pasadena, has already hosted same-sex weddings.

The 2003 ordination of Gene Robinson, the church’s first openly gay bishop, has caused some more conservative Episcopal churches to split from the national body.

It has also caused considerable tension in the worldwide Anglican Communion to which the Episcopal Church belongs.

Integrity is an affinity group for gay Episcopalians.

Another denomination whose churches sometimes welcome openly gay members is the Wisconsin-based American Baptist Churches in the USA, a much smaller group that the more widely known "Southern Baptists".

Even though the group's governing body, the General Board, declared that in 1982 that "homosexuality is incompatible with Christian teaching," some of member churches, including Seattle First Baptist on First Hill, welcome LGBT members.

That caused a ruckus in 2006 when many American Baptist churches in California, Hawaii, Nevada and Arizona broke away from the national church, saying the General Board failed to penalize Seattle First Baptist and the regional grouping to which it belongs, Evergreen Association, for welcoming gays.

The Southwestern churches also complained about the welcoming and affirming policies toward LGBT members by New York's Rochester-Genesee region.

The denomination's general board has refused to discipline the regions who accept LGBT-welcoming churches from outside their areas, citing Baptist freedom and local church autonomy as reasons.

Association of Welcoming & Affirming Baptists is an affinity group for gay Baptists

Presbyterian Church (USA) has not explicitly addressed the issue according to Pew, but prohibited the ordination of gays in 1997.

Issues concerning gay clergy and gay weddings church are expected to arise next week when the General Assembly holds its biennial meeting in San Jose.

The legislative body of the Evangelical Lutheran Church in America is expected to consider gay marriage and the ordination of openly gay ministers in 2009. A church task force in March recommended that the church continue to define marriage as a union between a woman and a man.

Lutherans Concerned is an affinity group for gay Lutherans.

Lutherans elsewhere in the world take a range of approaches.

In Germany, an openly gay man is one of two candidates chosen by a selection committee to become bishop in a northern city.

In Norway, where the official state church is Lutheran, the legislature last week approved marriages for lesbian and gay couples, but gave individual churches and ministers the option to decide whether or not to perform ceremonies for same-sex couples.

Denominations or groups that express clear condemnation of same-sex relationships include:

• National Association of Evangelicals which reaffirmed in 2004 a 1985 resolution that homosexuality is not sanctioned by the Bible. Thus the group does not support gay marriage or civil unions. Members of the Association are among the most outspoken opponents of gay rights.
  Evangelicals Concerned is an affinity group for gay evangelicals.

• Southern Baptist Convention issued a statement in 2003 confirming its opposition to gay marriage. It called on “Southern Baptists not only to stand against same-sex unions but to demonstrate our love for those practicing homosexuality by sharing with them the forgiving and transforming power of the gospel of Jesus Christ.”

• Roman Catholic Church has become even more vocal in its condemnation of same-sex relationships since Pope Benedict took over the Vatican. Even though US Catholic bishops have condemned violence and bigotry against gays and lesbians and said that they "must be accepted with respect, compassion and sensitivity," several bishops have taken lead roles in opposing any legal recognition of lesbian/gay couples and often oppose rights legislation designed to protect LGBT people from discrimination.
  Dignity is an affinity group for gay Catholics.

• Islam. Islamic law explicitly denounces homosexuality; same-sex marriage is prohibited in all Muslim countries, the only disagreement being the severity of punishment for what are termed “sinful and perverted acts.”

• Church of Jesus Christ of Latter Day Saints (Mormons) Mormon theology mandates that “marriage between a man and a woman is ordained of God.” As a result, the Church of Jesus Christ of Latter-day Saints does not endorse same-sex marriage.
  Affirmation is an affinity group for gay Mormons.

• Lutheran Church-Missouri Synod reaffirmed in 2006 its position that same-sex marriage is “contrary to the will of the Creator.”

• Orthodox Judaism -- Leaders have defined marriage as an institution between man and woman and therefore do not allow for same-sex marriage.

• United Methodist Church's top policymaking body reaffirmed that marriage is between a man and a woman in April.
  Affirmation is an affinity group for gay Methodists.

Full article: Religious Groups' Official Positions on Same-Sex Marriage | Pew Forum
Pew Forum surveys denominations' views on gay marriage | - Los Angeles Times

reference: PFLAG list of gay affinity groups for various denominations
A longer list of affinity groups by GayChurch.org
A list of "wecoming and affirming" churches worldwide from GayChurch.org