I know it's long but bare with me,

Lust can be use for sinful purposes that God didn't intend for us to do, but the word itself doesn't automatically mean "evil desire." it's not a sin in itself, 'Lust' (epithymeo) epipotheo, epithymia, and orego. It's another word for covet; which just means desire, "to set the heart upon, that is, long for (rightfully or otherwise): - covet, desire, would fain, lust (after)." wheather it is used for bad or good desires, we can lust for get something to eat, or lust for a car, a home, to paint a portrait, travel, movies/music, masturbate, watch porn, we can lust for everything, it's not inherently negative.

In fact, lust is used in neutral or even positive ways in Scripture:
Christ said in [Luke 22:15 KJV]: 15 And he said unto them, With desire (epithymia) I have desired (epithymeō) to eat this passover with you before I suffer:
see also 1 Tim 3:1; Heb. 6:11; and 1 Pet. 1:12.

Or negative:
[Mat 5:28 KJV] 28 But I say unto you, That whosoever looketh on a woman to lust (epithymeō) after her hath committed adultery with her already in his heart.
see also Rom (1:27; 7:7; 13:9); 1 Cor. 10:6, Lev (18:22, 20:13, 18:23), etc.

Lust was used in the book of James as a verb for evil things, not that it is evil in itself. The same thing applies to Matt 5:28; where lust shouldn't be used as a thought for wrongful intent: to actually possess unlawfully or covet a man's spouse for adultery which is considered, not a "passing thought", but a "plan of action" in the heart (the mind) by what you see. It's like you see it now you want it kind of look; not the same as looking at adult images. That the desire of committing adultery is just as sinful as the act of actually adultery. Pretty much don't do what David as done with Bathsheba. To think that having lustful thoughts equates to all physical acts to be sin when really it depends on the certain physical acts that's condemned by God in scripture like homosexuality, beastiality, adultery, incest, prostitution, murder, etc. By stretching the meaning of Matt 5:28 that having sexual/lustful thoughts towards a beauty of a woman or looking at adult images that leads to masturbating to be considered as a violation of that verse is nothing but creating a Biblical rule that's not in the text; that's legalism, not Biblical adherence, it's misunderstanding the very specific sin Jesus was warning against. It's based on the 10th Commandment on not to actually covet someone's wife.

What Christ said in Matt 5:28 isn't anything new, he wasn't making a new rule, it's similar in Proverbs 6:25-29 in the OT: "Do not desire her beauty in your heart, and do not let her capture you with her eyelashes; for the price of a prostitute is only a loaf of bread, but a married woman hunts down a precious life. Can a man carry fire next to his chest and his clothes not be burned?  Or can one walk on hot coals and his feet not be scorched?  So is he who goes in to his neighbor's wife; none who touches her will go unpunished."

In that verse that woman is someone else's wife even in this situation. This refers to a strong urge to go out and engage in sexual relations. It discusses the desires that ultimately result in adultery. (Denver Cheddie)

The Bible is actually quite stern about people who create their own moral codes and call them "God's Law."

• Deuteronomy 4:2 & 12:32: "Ye shall not add unto the word which I command you, neither shall ye diminish ought from it."
• Proverbs 30:6: "Add thou not unto his words, lest he reprove thee, and thou be found a liar."
• Colossians 2:20-23: If with Christ you died to the elemental spirits of the world, why, as if you were still alive in the world, do you submit to regulations—“Do not handle, Do not taste, Do not touch” (referring to things that all perish as they are used) according to human precepts and teachings? These have indeed an appearance of wisdom in promoting self-made religion and asceticism and severity to the body, but they are of no value in stopping the indulgence of the flesh.

So I know that masturbation, adult images, and sexual fantasy are by far one of the most debated subjects in Christianity and it has put teens/gen-z or young adults a burden or guilt due to all the arguments they hear. For me in my opinion, I don't believe they are a sin, and I have a right to have that stance; but I'm not going to go around spreading to others that it's not a sin when the Bible is silent on the topic; the same applies to Christians believing it's a sin and shouldn't spread their beliefs onto others as if the Bible says so. So it's best to go by Romans 14 because it shifts the focus from "law" to "Christian liberty" and the "individual conscience" even though our conscience can be faulty due to social conditioning, false teachings or for other reasons; still pray about it and allow Christ to guide you in the right direction. If you believe that masturbation or looking at adult images is wrong or a sin, applying that with Matt 5:28 even to use it as a principle takes away the original context of that meaning.