This issue comes up a lot in Christian circles and it's worth discussing here too, especially since RP thinking is mostly derived from these in the first place.

WHAT IS LUST?

When Jesus says in Matthew 5:28, "Anyone who looks at a woman lustfully has already committed adultery with her in his heart," he's using the words epithumeo for lust and moicheia for adultery. This is different from the word porneia that he gives as the exception for divorce, but that's beside the point and /u/BluePillProfessor already tackled that issue in a separate post. Moicheia is actual adultery, as defined in their culture (which is a little different from how we view it, but not too too far off).

But that word epithumeo does not mean "lust" in the way we think of the word today. It's actually the same word that Exodus 20:17 in the Septuagint (earliest known Greek translation of the OT) translates as, "You shall not covet [epithumeo] your neighbor's wife."

Biblically, the idea of coveting is not merely any old desire. It's an I would if I could mentality. If you see your neighbor's cow and think, "If I knew I wouldn't get caught, I'd totally steal his cow!" that's coveting. If you see it and think, "Dang, that's a great cow. I wish I had a cow like that. But I don't ... oh well." That's not coveting. That's desire.

Desire is healthy. Coveting is not. It's really that simple. If we were to stifle all of our desires and pretend we never wanted anything that anyone else has, we would totally lack all ambition in life and fail as a species. Paul saying things like, "Run in such a way to win the prize!" (1 Cor. 9:24) makes no sense if we're not allowed to desire something we don't yet have.

DRAWING THE LINE

Here's my simple test for discerning whether you're lusting or not:

  • Is the object of your desire not your spouse?

  • If there were no earthly hindrances to gratifying yourself with the object of your desire, would you do it?

If the answer to both questions is yes, then it's sinful lust. If the answer to either question is no, then it's not sinful.

EXAMPLES

Assume someone is just looking and not necessarily touching himself or anything (which de facto proves point 2).

  • She's only a character on the screen; she's not a real person; I can't have sex with a screen. If she stepped out of the screen and into your basement, would you bang her?

  • I don't know where she lives - it's probably the other side of the world. Okay, if she knocked on your door and walked in your bedroom, would you bang her?

  • I keep eyeing my girlfriend; she's getting me hot, but there are too many people around right now, which helps me keep control. If you were alone, would you have sex with her right then and there?

  • There's not enough time, so I'm just looking. If you got a free pass from all of your deadlines, would you then proceed?

  • You're alone with your GF on the couch making out. No one else is around, you have no deadlines, you're super into it, but you're afraid if you try to go further she might say no, so you exercise restraint. If you knew for a fact she'd say yes, would you bang her?

  • Suppose she would say yes and you're extremely confident of this, but you're afraid of what your mom might think if she found out. If you knew for a fact your mom would approve, would you do it?

  • Suppose everyone would approve of this, but you know you're not married and you don't want to ruin your sense of moral superiority. If you knew your sense of moral superiority could not be tarnished by sleeping with your girlfriend, would you do it?

  • Suppose everyone would approve, you have no worries or concerns about it, you're appropriately humble, bu tyou know that it's wrong to sleep with someone outsie of marriage because God said so. This isn't lust - it's appropriately self-controlled desire. Why? Because it's not an "earthly" hindrance.

  • Suppose you know God wouldn't want yo uto do it, but you also have the moral superiority thing going on, you're also afraid of her saying no, you're also afraid of what people would think if they found outo, etc., and nso in that situation you would refuse, and it is impossible to discern which of these hindrances would be primarily responsible for your refusal and you are not sure if "honoring God" alone would be enough (but it might be, you just don't know) ... *This is probably sinful lust. At the very least, you know you're being reckless with your thoughts because you don't know whether or not you would be able to restrain your desire if all of those other things were removed. As such, you're exposing yourself to temptation without knowing if you can resist on the right motives alone - and that itself goes against the grain of the command to "flee from temptation," rather than the repeated condemnation the Scriptures give to our efforts to justify our temptations and desires on some technicality.

CLARIFYING INTERNAL MOTIVATORS

As noted above, one of the "hindrances" can be internalized humanistic motivations. For example, "I'd feel really guilty afterward" is a hindrance that's usually not a righteous motivation.

This gets me a lot of heat in Christian circles, but I fully believe that all forms of feeling "guilty" are not from God - that Jesus came to free us from guilt. So, when pastors talk about "guilt" as a healthy reaction to sin that keeps us motivated to avoid sin, I usually object and say something like this:

No, trying to avoid guilt is a humanistic motivation that places our own internal emotional state as an ultimate priority. Our reason for living should be our faith, hope, and love. Romans 14 says anything that does not come from faith is sin. So, if your motivation is a desire to avoid negative feelings, you're really doing it for yourself, not for God, and that's sin - and that applies to all negative feelings, not just guilt. The better option is simply to eliminate this guilt and negativity from your life altogether. After all, you're already forgiven - why bother feeling guilty when God has already declared you "not guilty"? Then, live in that freedom out of appreciation for the verdict he rendered, not for fear that he might undo that verdict and somehow make you feel or be guilty again.

FINAL THOUGHTS

Sadly, many people think they're in "honoring God" mode, when in reality, if the circumstances were right, that alone would not be enough to hold them back. The temptation of having some porn star in your bedroom without consequence, without guilt, without anyone finding out, etc., and she's all over you ... "honoring God" simply wouldn't be enough of a motivator for as many people as would like to think it would be. As a result, many people force themselves into that last category in the hopes that creating ambiguity can give them some wiggle room.

Now ... the thing that gives context to all of this is that actual self-gratification in any of these circumstances proves that you actually would act if you had the opportunity because you are acting. So, if the thought leads you to beat off, then it's sinful lust, unless the object of your desire is your spouse ... in which case it would not be sinful lust (although there's debate over whether or not it would fall under some other kind of "sin" category).