I think perhaps we are not understanding one another, or maybe it's a matter of not having experience with what is being suggested? Maybe I don't explain well either.
I'm not suggesting that we try to game the system, though I recognize the temptation to do so.
Btw, I'm not suggesting that was your intent. But yes, as soon as we begin to talk in that way, at least some people will use the thinking in that way. Perhaps lack of maturity (which is not a sin, we all start somewhere) or poor teaching, or whatever, but some (or many) will, which is why I went there to caution against it, I suppose. Even starting out well, one can be led to become LESS mature if one is a bit lazy spiritually, by having things put forth before them in terms like this. But I did not mean to imply that that was your intent. My apologies.
It is a necessary part of understanding what is, and what is not sin that we consider where the line is between the two. I think the line depends on the individual and the circumstances, meaning it will be different every time and I believe this is where I really am promoting flexibility as opposed to the legalism you suggested.
And I think this is where we have a breakdown in communication.
I certainly agree that we need to know where sin begins. As to how much it varies between persons I can only think that there are circumstances where it would be acceptable to engage in thoughts of a sexual nature - but that would be for married persons, about their spouses, and in a non-objectifying, loving way. Those who are chaste and engaged will no doubt have the subject on their minds as well, but it is best for the sake of chastity not to give that free reign. Those who are not in a position to fulfill their desire, which is pretty much everyone else, or even married persons in an objectifying sense as Padiske noted, will cross over into sin as soon as they begin engaging the thoughts.
What really struck me though is that you see my statements as "legalistic" - no offense is taken, btw, but this is how I know that I must not be explaining well. I would say what I suggest is the far end of the spectrum from legalism, while what you propose strikes me more so (also no offense intended).
The key is that I am not suggesting "at what point do we incur sanctions" from God, whether that be need of forgiveness, punishment, and/or whatever one sees as the result.
Instead, I am asking - how does this affect the person? Are they becoming more like Christ? Are they further infecting themselves with the sickness of the world? What is the effect on their mind, their capacity to love God and others? Are they strengthening enslavement to base passions? (Passions having a particular meaning to the Orthodox and not meaning simply a strong or loving feeling, but rather a perverted - in the sense of having been changed - desire.)
It's not about crime and punishment for us, which I see as legalism. Rather, it is about becoming a whole, healed person in the likeness of Christ who is capable of giving and receiving love with both God and other persons.
As someone suggested over on the other thread where this question originally appeared, we can't become asexual through sheer will power and that we are designed to care about and think about sex. Even if it is only in terms of reproduction (as opposed to pleasure for the sake of pleasure) there will be some thoughts about sex. In other words, it comes across as unrealistic to suggest that any thoughts about sex, period, must be sinful. I don't think you are suggesting this, but I also think we don't have clarity on this issue of "sex thoughts" in general.
No, certainly I am not saying that all thoughts about sex are sinful. Sex was introduced by God and is blessed within marriage, a means of intimacy as well as the path through which children enter the world. Sex can and should be blessed. It is not inherently dirty or sinful, though we certainly have MANY temptations these days to warp it into that.
On the other hand, I think most people who have not tried would be amazed at how much control a person can gain, with the help of God, over all kinds of thoughts and feelings. Forgive me, but to say that it's only to be expected because we are human, while widely accepted, becomes an excuse for not trying.
If it is true that not any and all thoughts of sex are wrong, then where is the line drawn between the two? I would like to suggest that, in principle, the line will always be relative to the individual's personal motives. This is where careful self-examination as to where the line may happen to be, at any given moment, is critical.
I can't answer that without knowing how it would play out in a person's mind. And indeed, I'm not qualified to be a personal advisor on such things. I will say that you could be right, that motivation can be a factor, but I like to consider what I said above - that basically, if the sexual act would not be allowed, then thinking about it is sinful.
The other potential problem I see is that some people, in an exercise of examining their motives, could make one of several errors. The first I see is the potential to justify themselves through their motives. The second is that simply engaging with a thought if they ought not just keeps the door open longer and involves the person a bit more, and one who is undisciplined can easily slip back into fantasy while believing they are just thinking about the fantasy as a meta-thought. There are probably other potential pitfalls as well that I'm not thinking of. The human heart can be very deceitful ... having a plan in place to deal with thoughts before they happen is fruitful.
I believe this part of your post is consistent with what I've shared, except for one small difference. I think that, even if the act isn't committed, the person can still be guilty, based on motive. This is why, in my previous post, I gave the example of a person who wants to fornitcate, but only refrains from doing so because they fear being caught. What's happening in their heart is more important to God than what they actually do.
Then I haven't been clear, because I agree that a person can sin without the physical act.
The opposite can also be true, where a person fantasizes about fornication and, even if they could get away with it (meaning no consequences) they still would not do it, based on their personal integrity. Notice I said that it can be true and not that it always will be. The key is motivation, and only God and the individual can really know if they crossed the line from "thoughts about sex are not wrong in themselves" to "if I could I would" and even then many of us would probably be in denial over it.
I think this goes into more rabbit-trails than I can address right now.
But if I understand you right, I will say that one can fantasize about illicit sexual activity that they could get away with committing, but don't (for whatever reason) and still be sinning. They may show restraint by not fornicating, but they still did so in the mind, so it is still sin, and they still damage their person and their growth in Christ through their fantasizing.
But this mostly relates to fleeting encounters and this is where I feel your concern is probably most valid in that the argument could easily become justification for going around fantasizing about every person we see. In general I think it is a bad idea for people to include real people in whatever "thoughts about sex" they may have (especially when it's people they actually know in real life), precisely because, as you've suggested, temptation can easily become a slippery slope.
Well, including real people in fantasy is worse, I agree.
Simply, if a person is unmarried and has no outlet for sexual gratification, do you think it's a good idea for them to spend significant amounts of time fantasizing about sexual activity?
I have found the Freudian-based models that claim that such mental activity somehow diverts energy from actually committing the acts to be flawed. Rather, I would say (and this is strongly taught by our Church) that constantly feeding secret desires only keeps them alive and strengthens desire to act on them. I realize sexual thoughts are probably the most controversial of the possible topics here, but they work like any other desire.
This is why Paul advised people to marry.