By way of an answer, here is an extract from my book:

Biblically speaking, lust is always presented as something negative. Lust is about desiring something that is not yours to have. Lust is unhealthy desire. Lust is destructive because acting on it always means violating God’s laws and other peoples rights. In the end, lust leads to our destruction. Proverbs tells us that, “The righteousness of the upright delivers them, but the treacherous are taken captive by their lust.” And Jesus said that to look at a woman lustfully is to commit adultery with her.

Because lust is always negative, we need to learn to distinguish it from those desires that are appropriate and healthy. For example, it is right to desire food when you are hungry, drink when you are thirsty, and sleep when you are tired. These are desires God gives us in order to stay healthy. But it is wrong when these desires get distorted – when our desire for food becomes gluttony, or our desire for drink drunkenness, or our desire for sleep laziness.

In a similar way, it is right for a husband and wife to desire one another sexually, but it is wrong to have this desire towards someone you are not married to. Practically, this might mean that a husband carries a picture of his wife in the nude with him when he is away from home because he has a right to see and desire his wife. But carrying a picture of a woman who is not his wife would be lust, and wrong!

So where does this leave you in how you should feel about your boyfriend or girlfriend? Obviously, there is meant to be physical attraction between you. It would be really weird to have a relationship with someone if you didn’t fancy them! But where is the line that means you have crossed into sinful lust?

This becomes even more of a problem if you are going to get married, and the closer you get to the wedding day. I remember when Grace and I were engaged being given a book about sexual technique in order to help prepare us for when we were married. I didn’t know what I was supposed to do with that as all it did was make me think even more about Grace’s soft skin than I had been already – was I allowed to be thinking about her this way? As we were engaged was it ok to now start fantasizing about sex? Was I lusting sinfully or staying on the straight and narrow?

So how are we going to answer this question about lusting after your boy/girlfriend?

I think the first thing is realizing that it is lustful – and therefore sin – to desire anything that you are not entitled to. And the reality is, you are not entitled to the body of your boy/girlfriend. Until you have come into the covenant relationship of marriage with someone you have no rights over their body. You therefore need to be doing everything you can to kill lust – or it will kill you. In a boyfriend/girlfriend relationship this means not doing anything that stirs up passions in you that you should not be experiencing.

But perhaps an even more important thing than recognizing what lust is, is seeking to have a heart that runs faithfully in pursuit of Jesus over and above anything else. If your number one desire is to live in a way that pleases him, then lust is going to get squeezed out of your way of thinking. And this means that rather than lusting after the person you are dating you will be seeking Jesus as to how you can best honor him and bless them.

Having your heart directed this way doesn’t mean that you will no longer feel sexual attraction or desire, but it will mean that those desires are kept under control. It means that you will be able to shepherd those desires and keep them where they are meant to be, just like a farmer herds a flock of willful sheep so they all stay in the place where they are safe.

And if you do get married, having your heart directed in the right direction will mean that you are not bringing lust to bed, but pure and passionate desire for your marriage partner which you offer to them as a sacrifice of love – and not merely as a way to satisfy your own selfishness.