Lust is not sexual per definition.
Lust is also not sin per definition.
The Greek word (epithumio) is translated lust or covetousness and simply means strong intense desire. Thus lust could be a good thing when the strong desire is for good things. Lust becomes sin when it for having things one is not supposed to have.
16. “But blessed are your eyes for they see, and your ears for they hear;17. “for assuredly, I say to you that many prophets and righteous men desired (lust = epithumio) to see what you see, and did not see it, and to hear what you hear, and did not hear it.
Matthew 13:16-17
15. Then He said to them, “With fervent desire (lust = epithumio) I have desired to eat this Passover with you before I suffer;16. “for I say to you, I will no longer eat of it until it is fulfilled in the kingdom of God.”
Luke 22:15-16
16. I say then: Walk in the Spirit, and you shall not fulfill the lust of the flesh.17. For the flesh lusts (lust = epithumio) against the Spirit, and the Spirit against the flesh; and these are contrary to one another, so that you do not do the things that you wish.18. But if you are led by the Spirit, you are not under the law.
Galatians 5:16-18
4. by which have been given to us exceedingly great and precious promises, that through these you may be partakers of the divine nature, having escaped the corruption that is in the world through lust(epithumio).
2 Peter 1:4
28. “But I say to you that whoever looks at a woman to lust (epithumio) for her has already committed adultery with her in his heart.
Matthew 5:28
Please note:
ADULTERY (moicheuo) is sexual sin of married people and carries the death sentence under Moses regardless of circumstances.
FORNICATION (porneia) is sexual sin of unmarried people and is treated much less severely under Moses than adultery. Fornication followed by confession and restitution is forgiven readily under Moses.
In Matt 5 28 Jesus is speaking of “adultery”. Adultery per definition involves married people. Jesus is not talking about an unmarried young man having strong sexual desire for an unmarried young women. This is not sin per se.
Obviously, any desire that is obsessive and out of control of the Spirit is sin, but the fact that is sexual is not the definition of sin.
The issue with sexual sin is always that of theft. Are you taking sexual liberties that belong to another man, i.e. this is another man’s wife or future wife. If this is not someone’s wife or future wife then the sexual sin is much less severe. As mentioned before if neither the man nor the women is married is cannot adultery and thus not the sin Jesus is referring tin Matt 28
God will avenge fornication because it defrauds your brother
3. For this is the will of God, your sanctification: that you should abstain from sexual immorality;4. that each of you should know how to possess his own vessel in sanctification and honor,5. not in passion of lust, like the Gentiles who do not know God;6. that no one should take advantage of and defraud his brother in this matter, because the Lord is the avenger of all such, as we also forewarned you and testified.
1 Thessalonians 4:3-6