There are different marriage laws in all the states and different definitions of marriage in every religious tradition. In addition to this diversity, civil marriage rights in the U.S. have been significantly broadened during the last fifty years.
Civil vs. Religious Marriage
Unlike some religious definitions, civil definitions of marriage do not usually mention childbearing, sexual relations, living arrangements, or religious beliefs or observance.
When clergy or congregations marry couples it is a religious rite, not a civil ceremony, although the government may recognize it. Clergy and congregations choose whom they marry. They aren't compelled to accept the state's marriage definition, and indeed, many religious institutions don't accept it. Many religious institutions are more restrictive than the state, rejecting interfaith marriages or remarriages after divorce. And some have a broader definition, blessing the unions of same-gender couples.
There are a variety of views of the purpose of marriage in the religious community; some think gender is irrelevant. Many religious organizations and people of faith have definitions that are probably different from yours.
Some faiths consider marriage an aid to religious instruction. Some call it an expression of committed love. Others say it is mainly for raising children. In some Christian faiths marriage is a sacrament, in others it is not. But whether one agrees with someone else's definition of marriage (or baptism, or sacrament, or communion) one must respect everyone's Constitutional rights of free speech and free exercise of religion.
The Universal Fellowship of Metropolitan Community Churches, Ecumenical Catholic Church, Church of God Anonymous, ALEPH: Alliance for Jewish Renewal, Reconstructionist Judaism,
Reform Judaism, and Unitarian Universalist Association bless same-gender relationships as a matter of policy.
The United Church of Christ, and various Quaker groups leave the decision to clergy, congregations or local governing bodies.
The Presbyterian Church (USA) allows the blessings of same-gender unions with terminology restrictions.
Entanglement and Discrimination
The federal government and 49 state governments refuse (on what appear to be purely religious grounds) to grant the same legal recognition to same-gender couples that is available to mixed-gender couples who meet the same qualifications. State governments generally let religious ceremonies double as state ceremonies (in some states it is illegal to call a religious blessing a marriage without benefit of a marriage license from the state); the civil definition of marriage covers almost all religious definitions. (Some are more restrictive than the government.) Same-gender relationships are blessed in houses of worship by clergy and congregations from a wide range of religious traditions. The Constitution guarantees religious liberty. but same-gender marriages are still not recognized by almost all U.S. governments.
Civil marriage law has historically been used to legally encode segregation and majority privilege; it was forbidden to members of certain ethnic groups; and forbidden between people who were not members of the same ethnic group (blacks and whites still could not marry each other in some states until the Supreme Court overturned those laws in 1967). Denying legal marriage recognition on the basis of a single characteristic makes it easy to discriminate against everyone who shares that characteristic.
Without civil marriage, families are not legally recognized. In situations involving child custody, medical decisions, burial, and inheritance, preference is given to legally-recognized family members. There are thousands of local, state, and federal laws in which legal marital status determines government treatment of two consenting adults in a relationship. Without legal marriage recognition it is difficult or impossible to legally establish and maintain families(especially when facing opposition from relatives), contrary to the Constitutional right of free association.
Today, many see marriage as the last line of defense for encoding the supposed supremacy of heterosexuality. Only same-gender couples are legally discriminated against in civil marriage in the U.S. States have different requirements for marriage (such as residency and waiting periods, age of consent, kinship restrictions, and blood tests), and different procedures for obtaining a license, but all states and the federal government automatically recognize all marriages and divorces from all states, except those of same-gender couples. Within the last few years "Defense of Marriage" acts have been passed by Congress and more than half the state legislatures. But civil marriage isn't jeopardized by opening it up to more people. There aren't a limited number of licenses; people who support civil marriage for same-gender couples are not asking the state to stop recognizing mixed-gender marriages.
Religious Support for Equal Rights
Many religious organizations, including some that do not recognize religious same-gender marriage, either directly support civil marriage for same-gender couples, support equal rights for same-gender couples, or are opposed to the denial of equal rights for same-gender couples. These include ALEPH: Alliance for Jewish Renewal, American Friends Service Committee, California Council of Churches, Central Conference of American Rabbis, Church of Religious Science, Ecumenical Catholic Church, Hawai'i Council of Churches, Interfaith Working Group, Pacific Congress of Quakers, Presbyterian Church (USA), Reconstructionist Rabbinical Association, Unitarian Universalist Association, and Universal Fellowship of Metropolitan Community Churches.
The reasons religious organizations support equal rights for same-gender couples are varied. But it is fair to say that most see it as a matter of love, justice, basic fairness, and civil rights. Many agree that legal recognition of same-gender marriage would make very positive moral and social points--that we as a people value committed, caring relationships and do not discriminate on the basis of gender, sexual orientation or religion.
Freedom to Marry is the gay and non-gay partnership working to win marriage equality nationwide. Headed by Evan Wolfson, one of America's leading civil rights advocates and lawyers, Freedom to Marry brings new resources and a renewed context of urgency and opportunity to this social justice movement. Freedom to Marry brings the work of its partner organizations and their many approaches — litigation, legislation, direct action, and public education — into a larger whole, a shared civil rights campaign that fosters heightened outreach to non-gay allies.
Freedom to Marry encourages dialogue with Americans thinking through the need to end discrimination in marriage, provides support to targeted state and local efforts, and promotes fairness for all families, including same-sex couples and the children raised by gay parents. By working to secure equal access to marriage, we help reinforce our country's historic commitment to freedom, the pursuit of happiness, and equal justice for all.
As we advance toward full marriage equality, we promote gains along the way for all families, including same-sex couples and the children raised by gay parents. By working to end the exclusion of same-sex couples and their families from marriage, we help reinforce our country's historic commitment to freedom of religion and the separation of church and state. By ending sex discrimination in marriage, much as we ended race discrimination in marriage a generation ago, we are building a better America, protecting and supporting families, children, and the freedom of choice for all.