Christians need to be equipped to understand what the Bible teaches about homosexuality.
Two recommended books on the subject are Kevin DeYoung's What Does the Bible Really Teach about Homosexuality and Robert Gagnon's The Bible and Homosexual Practice (both linked below).
DeYoung's book is an excellent overview of the subject written to a general Christian audience. He covers all the major passages and responds to common objections. If you read only one book on the subject, make it this one.
Gagnon's book is widely regarded as the go-to book on the subject at a scholarly level. It would be more than most people would want to read through, yet it is an excellent reference book to have on hand. For those who really want to delve into hermeneutical issues, this is a must-read.