Q: In light of all the recent publicity about same-sex marriage, where does the LCMS stand on the issue?
A: God gave marriage as a picture of the relationship between Christ and His bride the Church (Eph. 5:32). Homosexual behavior is prohibited in the Old and New Testaments (Lev. 18:22, 24, 20:13; 1 Cor. 6:9–20; 1 Tim. 1:10) as contrary to the Creator’s design (Rom. 1:26–27). The LCMS affirms that such behavior is "intrinsically sinful" and that, “on the basis of Scripture, marriage [is] the lifelong union of one man and one woman (Gen. 2:2-24; Matt. 19:5-6)” (2004 Res. 3-05A). It has also urged its members “to give a public witness from Scripture against the social acceptance and legal recognition of homosexual ‘marriage’ ” (2004 Res. 3-05A). At the same time, the Synod firmly believes "the redeeming love of Christ, which rescues humanity from sin, death, and the power of Satan, is offered to all through repentance and faith in Christ, regardless of the nature of their sinfulness” (1992 Res. 3-12A). The Synod developed a Law/Gospel ministry plan for use in ministry to those who are troubled by homosexual desires, Ministry to Homosexuals and Their Families.
The Synod's Commission on Theology and Church Relations has prepared a report titled Human Sexuality: A Theological Perspective that discusses the issue of homosexuality on pages 32-36. Another resource addressing the matter of same-sex relationships available online is the document Theological Implications of the 2009 ELCA Decisions.
There is additional information found in the Frequently Asked Questions on Homosexuality.
http://www.lcms.org/faqs/lcmsviews#samesexmarriage