What is sin?
Paul tells the Romans that: “for whatsoever is not of faith is sin” (Rom 14:23). James tells the believers: “Therefore to him that knoweth to do good, and doeth it not, to him it is sin” (James 4:17). John says: “for sin is the transgression of the law” (John 2:3). He amplifies it later by saying: “All unrighteousness is sin” (John 5:17). Take note of the theme of predilection: falling short. In others words sin is falling short of accomplishing the law.
What is lust?
Lust is the strong desire or craven for the forbidden. That means you have God here and the devil on the other side. “For all that is in the world, the lust of the flesh, and the lust of the eyes, and the pride of life, is not of the Father, but is of the world (I jn 2:16-17).
Since we were made to worship God (Eccl 12:13) then our desire should be towards God. But if you love God and love the devil then you are lusting after the world. As you lust after the world, you start to conform to the world to a point where you mingle and are spotted together. This then makes you a friend to the world and an enemy to God and thenceforth lust conceives and gives birth to sin. We become closer to our desire that our righteousness does not exceed that of the Pharisees and scribes.
The relationship between lust and sin is that lust conceives sin (James 1:15). That means: lust on its own is not sin but a gateway to sin. Just by seeing a beautiful girl and lusting after her is not a sin. Your first question would be then why did Jesus say: “but I say unto you, that whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart” (Mt 5:28). What Jesus is saying is that if any man would look at a beautiful woman and allow his desire to take control over him then he has sinned already. If not by the same Bible saying: “Then when lust hath conceived, it bringeth forth sin: and sin, when it is finished, bringeth forth death” then we are left in some contradiction.
Even when lust conceives sin, it is merely lying on our door (Gen 4:7). Except we open the door it will not come in. Even in the OT days where worship was physical the sin of ignorance was not forgiven (Lev 4:2 Lev 5:15). In Eph 4:26 the Bible says let us be angry but let our anger not lead us into sin. How can anyone be angry and not sin? That means anger by nature is not sin.
How to deal with sin:
Timothy tells the people that the best way to deal with sin is to rebuke those who sin openly (I Tim 5: 20). That does not reject “Brethren, if a man be overtaken in a fault, ye which are spiritual, restore such an one in the spirit of meekness; considering thyself, lest thou also be tempted” (Gal 6:1). But if they refuse to be restored then follow the guidelines of I Corinthians 5:9-13. Let us exhort one another daily. In summation, when someone sins, rebuke them openly, try to reinstate them back, if they refuse restoration then excommunicate them from fellowship. But daily exhort each other.
How to deal with lust:
The only way to avoid lusting is to walk in the Spirit (Gal 5:16). That way you will worship God in spirit and in truth.
Until then, lust is not sin.
Go for the person who loves you most and not the one you love most for the one you love most may not give you the love you need most because though love is blind marriage is an eye-opener” (Hamilton Ayuk).