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God has given us His instruction on how we are to handle our desires. The fact that we do not follow those instructions is not His fault -- all of us would be so much better off and blessed by God if we did follow His instructions and obey Him, not only in this, but in all things.
Our thoughts (whether those of teenagers or adults) are bombarded by the world, all designed to encourage us to sin. Our women are dressed immodestly (especially teenagers), and TV and other forms of media, especially advertising, are laden with sex, so yes, it is very hard in today's world not to sin with our minds. It is true that impurity in our thought life is a sin, but it is a lesser sin than when we compound it by actually acting on those thoughts. Acting on our thoughts to actually commit the sins of immorality, promiscuity, and adultery is significantly worse and very much within our control.
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