Even if we believe homosexuality is against God, I don't believe we should speak out against it unless we're asked. It's not our responsibility to go around pointing fingers at people or condemning people. If we're asked, we should give an honest answer, give our reasons, and leave it at that.
Jesus showed love and mercy to everyone, no matter whom they were or what they were doing. He never went around seeking people to condemn. Even though He, Himself, was often confronted by the scribes and Pharisees, He never replied to them in an accusing manner even though He knew what they were doing was wrong. He merely answered their questions truthfully and let them come to their own conclusions.
The scribes and Pharisees lived their lives by the letter of the law. Their idea of serving God was so rigid and "by the book," there was no room for love. They went out of their way, trying to keep every law and commandment perfectly. When they saw anyone doing even the tiniest thing they thought was against the laws and commandments, they were quick to condemn them. Of course, Jesus was their favorite target because everything He did was based on love and mercy. To them, He seemed to be breaking the laws all the time. They just couldn't comprehend that "loving thy neighbor as thyself" was the actual fulfilling of the law. (Galatians 5: 14)
There are many Christians today who believe if they met someone and found out he was a homosexual, it would be their duty to point out to that person what a deplorable sin it is and that they need to repent. I just can't help but wonder how they could even think that kind of approach would make the person want to change his lifestyle or believe in a God who condemns. Yet that's exactly the approach far too many Christians use today. I think we, as Christians, need to seriously ask ourselves if we're following Jesus' example or are we trying to keep every law and commandment so perfectly, there's no room for love?
Homosexuality has been around since the beginning of time. The only difference today is its out in the open and even flaunted, which makes it a little harder to bear. Even so, it's not our place to condemn. Not speaking out does not mean we're condoning sin. We are to be an example to the world and our actions should speak louder than any words ever could. Our lifestyle should reflect Jesus' love and mercy, not the Pharisees' contempt and condemnation.