But each person is tempted when they are dragged away by their own evil desire and enticed. Then, after desire has conceived, it gives birth to sin; and sin, when it is full-grown, gives birth to death.
Following God’s plan for sex is about much more than just not going too far with your boyfriend or girlfriend. It is about purity. And the purity that God has called us to starts in the mind. Unless you can win the battle for mental purity, you won’t succeed at staying physically pure. Doug Fields talks about the fact that temptation begins in the mind. “A lot of people think that their strongest sexual organ is below their waist but it’s not. Let me tell you where your strongest sexual organ is, it’s your mind. A lot of us just think that sex is a physical thing. Sex is a mental thing as well.”*
Jesus makes this clear in the above passage of Scripture from His Sermon on the Mount. (It is important to note that in this passage Jesus is exaggerating to emphasize his point. He is using a literary device called hyperbole. Please do not go poke your eyes out, okay?) It is clear from His statement that what goes on in our minds is very important to Him. He says that lusting after a woman is just as sinful as having sex with her. But what is lust? Technically lust is simply a strong desire for something, but in this context we understand Jesus to mean it as longing for someone sexually, to think sexual thoughts about someone you do not have the right to think about that way. So if you are thinking sexually about someone to whom you are not married then you are lusting and it is a sin.
Looking at James 1:14-15, I think it is clear that the purer our minds are the easier it will be for us to control our bodies. So here are some helpful hints on how to keep your mind pure. First, you can make things easier on yourself by avoiding sexual content. If the TV shows and the movies and the music you listen to are constantly talking about sex, then guess what? You are going to think about it constantly and it is going to be hard for you not to lust. So make it easier on yourself and avoid putting this stuff into your head.
One thing I have tried to do to maintain my mental purity is that I don’t watch movies that have nudity in them. I know that I can’t look at naked women without lusting (I don't really think any straight man can) so I made the decision to not watch movies that have nudity in them. Fairly often a new and awesome action movie comes out that I want to see but I go to imdb.com or pluggedinonline.com and see if it has nudity. If it does, then my wife and I don’t watch it. Have I missed watching some movies I wanted to see? Yes, but it was worth it because I prevented myself from falling into sin. I think this is exactly the type of thing Jesus is talking about when He says "if your right hand causes you to stumble, cut it off." So don’t watch movies with nudity, don’t listen to music about sex, and don’t watch TV shows about it. Make your own life easier.
Second, when you aren't sure if you are lusting use the 'three looks and you're lusting' rule. I know it is cheesy but I learned it when I was in seventh grade and it has proven to be helpful for me. Let’s say that you see a beautiful (but scantily) clad woman or man walking down your street. The first look was unintentional. You couldn't help it. You were just out watering your lawn or whatever and they walked by. But sometimes, almost involuntarily, you look a second time to make sure you saw what you thought you saw the first time. You might say to yourself, was that person as hot as I think they were? You turn around and glance back only to realize that they were. But now you have a choice. You can turn around and stare at the person and try to take every part of their body in with your eyes or you can choose by God’s strength not to. If you choose to look that third time, or if you choose to entertain thoughts about that person then you are lusting. Usually you are okay with that first or maybe even second look. It’s the third look that gets you. (Just to clarify these "looks" are glances. You can’t stare at someone for 2 minutes and say, “Oh, it’s okay I am still on my first look.”)
I want to close with a brainteaser. If I put a container in front of you and told you that your task was to get all of the air out of that container, how would you do it? Now it is very possible that you are smarter than I am and know of some sciency (oh it's a word) way to do this but the only way I know of is to fill the container all the way full with something else, e.g. water. The water would completely fill up all the empty space and push the air out. It can be the same with our minds. The best way to maintain purity is not merely to avoid putting bad things in our minds but to also fill our minds with pure thoughts. Philippians 4:8 says, "Finally, brothers and sisters, whatever is true, whatever is noble, whatever is right, whatever is pure, whatever is lovely, whatever is admirable—if anything is excellent or praiseworthy—think about such things."
For further reading...
* Doug Fields from "Best Sex Ever" series