I think you are confusing women with men. Men generally fall into instant lust mode (in wanting to have intimacy or to look at a woman's body (or body parts) in a lustful or desirable way) if they allow themselves to look at a beautiful woman in a lustful way within a matter of a second, and it takes time for women to stir up to be fully lustful (to want to have intimacy or sex) towards a guy. So let me get this straight, you think that a woman being stirred up to have lust towards a guy's body (not sexual fantasies yet) (as long as it was something they did not originally desire or intend) is okay vs. having sexual fantasies? Meaning, are you saying that lusting after body parts is okay if it was non-intentional (Meaning, a handsome guy just appeared out of nowhere within the grocery store in front of Martha within her view with big muscles and she could not help but to desire his body lustfully)? I do believe the admiration of a body part (or the whole body) in a lustful or desirable way is just as bad as sexual fantasies. Why? Because one is desiring to have something that is not in their own possession. The initial desire of body parts in a lustful way is with the same intent as a sexual fantasy. One has to completely not look at a person in lust in any way in order to not have adultery in their heart towards that person. For the desire of a body part is with the lead up towards intimacy or sex. It's wrong to desire a another person's body (body parts) lustfully if they are not the body of your spouse (even if it is not sexual fantasies yet). Paul says in order to avoid fornication, let every man have his own wife.
Jesus said:
“But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart. And if thy right eye offend thee, pluck it out, and cast it from thee: for it is profitable for thee that one of thy members should perish, and not that thy whole body should be cast into hell. And if thy right hand offend thee, cut it off, and cast it from thee: for it is profitable for thee that one of thy members should perish, and not that thy whole body should be cast into hell.” (Matthew 5:28-30).
Listen carefully to the words of Jesus above. It says whoever looks upon a woman to lust after her has commited adultery already in his heart. It does not say sexual fantasies (Although I am sure can include that). Lust is simply desire. To look at a woman with desire (of even their body or body parts in some way) is to commit adultery with that person in one's heart. Jesus says that a person can be cast into hellfire for this kind of sin. For the desire of a person's body part in a lustful way is with the ultimate intent of sex behind it (even if no sexual thoughts have not emerged yet).
Coveting is a sin, right? But what is coveting?
Coveting is desiring something that is not your own.
Would that not include desiring or coveting the body part of another that is not your spouse? I would say... “yes.”
Romans 7:8 (BSB) says, “But sin, seizing its opportunity through the commandment, produced in me every kind of covetous desire.”
But even if this was not the case, many Christian men do struggle with having sexual fantasies and they are in danger of hellfire unless they repent of such a sin. It may be a difficult task for many Christian men to do this but with God all things are possible. They have to keep trusting in the Lord and employ those methods that God has given to them to overcome. Sadly, most today do not feel like they have to overcome because they believe they are saved by having a belief alone in Jesus, and it is in nothing that they do. Many Christians today read Jesus' words in Matthew 5:28-30 as being a metaphor or they say his words in this passage only applied in the Old Covenant before the cross. In other words, people will do whatever it takes to re-write the words of Jesus because they do not line up with a belief they want to be true.
Before, you appeared to imply that one sin cannot keep a believer out of God's kingdom. But I believe one sin can make it possible for a believer to not inherit the kingdom of God. Paul says be not deceived, the unrighteous will not inherit the kingdom of God. Then Paul lists the kind of people who will not make it; Like: fornicators, idolaters, adulterers, effeminate, abusers of themselves with mankind, thieves, covetous, drunkards, revilers, and extortioners (See: 1 Corinthians 6:9-10).