The 32nd General Assembly of the Presbyterian Church in America, meeting in Pittsburgh, Penn., adopted a statement on Marriage and Sexuality at its meeting on Thursday, June 17, 2004. This statement was approved in response to an overture from Missouri Presbytery asking the General Assembly to address the proposed Federal Marriage Amendment.
Therefore be it resolved that the 32nd General Assembly:

a) Humbly call on the civil governments of Canada and the United States and all nations of the earth to act within their lawful powers and use whatever legislative and judicial instruments they deem most useful to ensure that marriage is legally defined and interpreted throughout their jurisdictions as existing exclusively between one man and woman. In support of this call the Presbyterian Church in America declares to the Church, the Nation, and the World that:

God is truth.

God's truth is firmly rooted and grounded in His immutable Nature and Being; it is not a construction of men; it is not variable, nor relative, nor dependent upon social or cultural context.

God has made His truth known to human beings most necessarily in the Holy Scriptures.
Among the truths that God has declared to human beings is the truth that the institution of marriage has been created by Him, from the time of the creation of human beings, and that it is ordained and defined by Him to be the lifelong union of one man and one woman (Genesis 1-3; Matthew 19:5-6).

God’s ordinance concerning marriage is binding upon all human beings in all places and at all times; it cannot be altered by legislative, judicial or cultural action. It is possible to deviate from God’s ordinance, but it is not possible to change it.

God has ordained civil authorities “under Him, and over the people, for His own glory and the public good” (Westminster Confession of Faith 33-1), to the end that good (as defined by Him) may be encouraged and defended and evil (as defined by Him) may be suppressed and punished.
"God is not mocked, for whatever one sows, that will he also reap" (Galatians 6:7). God will hold those whom He has placed in authority accountable for their actions, and He has declared:

b) Call upon the Presbyteries and Sessions of the PCA to strengthen the marriages in its own churches and exhort its elders to be bolder and more caring shepherds of Christ’s flock that we might help stem the scandalous rising tide of divorce in the church.
c) Call upon the Presbyteries and Sessions of the PCA to encourage all men and women, boys and girls within the PCA to live chastely for the sake of the Savior who bought them, whether in marriage or in singleness, whether they must do battle against heterosexual or homosexual temptation in seeking to be faithful to their Lord who loves them.
d) Call upon its members to be "the salt and light of the earth” in this context by exercising their full responsibilities as citizens including:

e) And that these “calls” be communicated by the [PCA] Stated Clerk to all who have inquired and may inquire concerning these matters.

The overture from Missouri Presbytery to the PCA General Assembly stated the following that led to the statement above:

Whereas the Church of Jesus Christ is sent into the world as salt, light, and witness to the full counsel of God; and

Whereas the Church—specifically, any branch thereof, including the Presbyterian Church in America (PCA)—is called upon at particular, extraordinary times to speak prophetically and serve compassionately with regard to particular public issues; and

Whereas the PCA has set precedent for boldly stating the demands of biblical justice for the civil commonwealth, while at the same time refraining from endorsement of particular legislative strategies, for example in its 1978 statement against legalized abortion; and

Whereas we the PCA, at this particular time and in the particular socio-political settings of North America into which God has sent us, are encountering much confusion over sexual matters in both private conversation and in the current public debate about so-called “gay marriage”; and

Whereas we should seize the opportunity to speak into this void, saddened that there has been so much erosion of the once-strong moral consensus that marriage is the divinely instituted, complementary human relationship of one man and one woman legally joined in a life-long, potentially procreative, monogamous commitment and that such family units are foundational for a healthy society; and

Whereas while the fundamental civil rights for all citizens also is foundational for a healthy society, the battle for “gay rights” is more and more being wrongly championed as the logical sequel to the fights of racial minorities and the fight for the rights of women, in spite of the fact that a person’s homosexuality is a characteristic fundamentally different than one’s ethnicity or gender; and

Whereas the real nature of homosexuality is obscured by the current preoccupation with the question of legal rights such that:

Whereas homosexuality, being unnatural desire, is not something to be celebrated but something from which Christ wants to redeem people by the great healing and restoring power of his grace (Romans 1:24-27; I Corinthians 6:9); and

Whereas it is not the promotion of human freedom but a form of cruelty, in the final analysis, for a society to encourage its citizens in the pursuit of desire which is, in essence, an affliction, and behavior which is offensive to God; and

Whereas certain civil magistrates have petitioned our church, wanting to know its mind on how marriage should be legally defined in the civil commonwealth of the United States; and

Whereas it is right and proper that the church should respond to such queries by bearing witness to the biblical truth that civil laws and courts should promote and protect marriage as a unique, male-female, monogamous community of two because of the way it propagates the human race and benefits the public in so many other ways.