Dr. Robert Gagnon has done phenomenal work on the issue of a biblical perspective on homosexuality. He is the author of
The Bible and Homosexual Practice (Abingdon Press, 2001) and his
website is full of articles of varying lengths and sophistication--beginning level overviews to meticulously documented professional essays. The following link is to a short essay entitled
Why "Gay Marriage" is Wrong. This short piece nicely covers some of the basics. Those wanting more information are encouraged to check out Dr. Gagnon's site.