When Lust is fulfilled

 It seems lust becomes sin when it is "fulfilled."

There must be a point when the lustful thought reaches the "fulfilled" stage, for it is that stage that walking in the spirit keeps you from: "I say then: Walk in the Spirit, and you shall not fulfill the lust of the flesh." Galatians 5:16.

So maybe one can sense the lustful inclination, but you don't act on it in your imagination or behavior, if walking in the spirit. 

Walking in the spirit must involve taking every thought captive in obedience to Christ  (2 Corinthians 10:5) and being energized by a renewed mind (Ephesians 4:23) in the truths of one's salvation and the glory of Christ (2 Corinthians 3:18).

For the weapons of our warfare are not carnal but mighty in God for pulling down strongholds, ⁵ casting down arguments and every high thing that exalts itself against the knowledge of God, bringing every thought into captivity to the obedience of Christ.

2 Corinthians 10:4-5

That you put off, concerning your former conduct, the old man which grows corrupt according to the deceitful lusts, ²³ and be renewed in the spirit of your mind, ²⁴ and that you put on the new man which was created according to God, in true righteousness and holiness.

Ephesians 4:22-24

But we all, with unveiled face, beholding as in a mirror the glory of the Lord, are being transformed into the same image from glory to glory, just as by the Spirit of the Lord.

2 Corinthians 3:18