So just a general question-- what is the biblical definition of lust? When does it go from just "thinking someone is attractive" or even being physically attracted to someone to lusting after them.
The main verse people are thinking of when they use the term "lust" is here:
Matthew 5:27-28 said:
You have heard that it was said, Do not commit adultery. 5:28 But I say to you that whoever looks at a woman to desire her has already committed adultery with her in his heart. 5:29 If your right eye causes you to sin, tear it out and throw it away! It is better to lose one of your members than to have your whole body thrown into hell. 30 If your right hand causes you to sin, cut it off and throw it away! It is better to lose one of your members than to have your whole body go into hell.
The two crucial words here are "adultery" and "lust". Adultery can only happen when one of you is married.
Lust is any strong desire where you set your heart on something, and would get it if you could. It once had this meaning in English, but in modern English the meaning has changed, it now means something sexual and negative. German still uses the term "lust" more like the older English, but for desires that are not necessarily as strong. I once had a woman ask me the equivalent of "do you have lust to go on a picnic with me?", and in German this meant simply, "would you like to go on a picnic with me?"
The King James used the term "lust" where the NET translation (above) uses the phrase "to desire her". There's a reason for this - the Greek word can be used for many kinds of desire. For instance, Jesus "eagerly desired" to share the Last Supper with his disciples - it's the same word.
The same word is also used in the Septuagint for Exodus 20:17, part of the Ten Commandments:
Exodus 20:17 Thou shalt not covet thy neighbour's wife; thou shalt not covet thy neighbour's house; nor his field, nor his servant, nor his maid, nor his ox, nor his ass, nor any of his cattle, nor whatever belongs to thy neighbour.
I think that it's misleading to use the word "lust", because most of us don't think of lusting after someone else's house or field. In the meaning of the original Greek word, I think that you are "lusting" when you say, "I want that!" It doesn't have to be something sexual. You can want to be close to God. You can want to be obedient.
I think anyone who heard Jesus and knew the Ten Commandments in the Septuagint would have been reminded of so I would personally translate this word "covet" in Matthew 5:28:
Matthew 5:28 But I say to you that whoever looks at a woman in order to covet her has already committed adultery with her in his heart.
But "desire" is also a good translation.
Back to your original question: I think it's helpful to use the term "covet". Thinking someone else is attractive is not coveting them. Thinking you want them, and would get them if you can, is coveting them. Being physically attracted to them is not coveting them - growing up in the country, we used to say "you can't keep the stray dogs out of your yard, but you can decide whether to feed them." If you feed the dogs, I think that's coveting. My parents use the three second rule - my father is allowed to look for three seconds. That's long enough to notice, but not long enough to start feeding the dogs.
What about couples? How do you differentiate between loving that person and feeling physically attracted to them and lusting. Would feeling sexually attracted to someone you were dating be wrong in and of itself, or only when you indulge those thoughts/feelings or act on them?
If you're dating someone and it's going well, you will be physically attracted to them, and you'll say to yourself, "I really want him!" (or "her", in my case)
That's the way it's supposed to be. If you don't have that feeling, I'd be concerned. That doesn't mean you can go ahead and have sex before marriage, boundaries are important, but of course you'll really want anyone you are in love with.
Hope this is helpful ....
Jonathan