Because their faith is in Christ's righteousness and not in their own. Becoming a Christian may or may not change your behavior right away, but it does change your identity. An understanding of that identity, and growing in dependance on God can have a dramatic effect on your behavior, but the flesh can never be righteous. It will always fall short of the glory of God. Those who claim that they do not sin in their bodies are liars.
Claiming that something is not a sin when the Bible makes clear it is, is evidence that God is not in them. Though behaviors may not change right away, a Christian will still recognize that their behavior is sin, and be thankful to God that He has forgiven them of all sin, and cleansed them from all unrighteousness. They will not call what is sin, not sin.
He does require perfection of the spirit, which is our true identity. Perfection is a gift, just as righteousness is a gift. You cannot justify sin, just because you cannot do anything but sin in your body. A homosexual may find it difficult to stop the practice (especially since it is tolerated), but there is no way if they are in Christ that they will say that it is not sin. If they do, they are liars, and not saved.