First off, thoughts "ARE." As a college teacher in small-town Mississippi, I am hypersensitized to "I does, he do."
Second off, it depends on the exact definition of sin. In the language of the New Testament, sin is αμαρτία, which literally means "missing the mark." If we view ourselves as ones that are supposed to be "holy, just like I [God] [am] holy," then, of course, any thought that "misses the mark" of absolute perfection is sinful.
But then, were exactly is the mark of this absolute perfection, do we know it?
If I think, "oh, my, this gal is beautiful" - does it mean that I paid my due to the wonderful job God has done creating beautiful females, - or does it mean that I am lusting for her?
If I think, "ah, the actor who played Hamlet in the play I watched yesterday was just super-astonishing" - does it mean that I am, again, paying my rightful due to a person who does his job "wholeheartedly, as if... serving the Lord, not men," expressing, through art, the everlasting truth of faith, hope, and love - or, rather, that I am "loving the world, and everything that is in the world - the cravings of the flesh, the lust of the eyes and the boasting of what I have and do?"
Christianity does not give one-size-fits-it-all, carved-in-stone answers. Everything you do, say, and think will be, and is already being, evaluated by God Who is the only One who "looks in the heart."