A christian is someone that believes in the teachings of Jesus Christ, namely salvation and sanctification. We come to salvation by believing Jesus Christ is God and his teachings are absolute, and that he is the Word of God, the Bible. We come to sanctification by striving daily to know God more, which happens through the indwelling Holy Spirit we receive during salvation and happens by reading and applying the Word of God in our soul, and then outwardly through our body.
Being homosexual or lusting after a women in your mind are equally damning, but rejecting these thoughts and actions and leaving them at the Cross is the message of the Bible.