The Old Testament has a lot of sins that people violate every day. That was the purpose of it. To show that man could never follow all those rules and live a sin free life. It showed that if they wanted to follow God, they would need the Messiah.

Almost correct. Yes the law was to demonstrate that people needed the Messiah, but that doesn't mean that what was sin then is no longer sin. While we are no longer required to adhere to the laws pertaining to worship practices and government because of the crucifixion, we are still under the “moral” law, where homosexuality would fall. It is still sin and therefore in conflict of living the Christian life.

However, (and this is a big “however’) so are many other things that Christians seem to overlook while targeting on homosexuality. This attitude of Christians is wrong and tends to undermine our purpose. "Living the Christian life" is not the prerequsite for salvation (faith is), therefore, not being a homosexual is not a prerequsite for salvation. In fact, I am sure that many Christians struggle with homosexuality, as we all are struggle with sin.