I want to pass along a helpful article by Tim Keller, a pastor and writer I'm appreciating more and more. He is the pastor of Redeemer Presbyterian Church in New York City (which is not exactly the easiest place to be a conservative evangelical!). In the article he takes on a common accusation leveled against Christians who say homosexuality is a sin: because you say you believe the Bible, and because the Old Testament says to execute those who engage in homosexual sex, then why don't you still obey those laws? Why pick and choose which parts of the Bible you will obey and which you will not?
His treatment is good because he understands the concept that while we do believe the Bible is true, some truths (read: commands) have an expiration date because the kind of relationship God has with the church is very different from the kind of relationship God had with Israel. The nature of God's relationship with "his people" is fundamentally different after Jesus. We call this the "new covenant" (which, by the way, is what "New Testament" means).