27"You have heard that it was said to those of old, "You shall not commit adultery.' 28But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart. 29If your right eye causes you to sin, pluck it out and cast it from you; for it is more profitable for you that one of your members perish, than for your whole body to be cast into hell. 30And if your right hand causes you to sin, cut it off and cast it from you; for it is more profitable for you that one of your members perish, than for your whole body to be cast into hell.
First of all, I believe that this instruction that Jesus is giving is for women as well as men. But is he saying that if you see someone of the opposite sex and feel a strong attraction to them that you are committing adultery?
Note, today's verse doesn't say "whoever looks at a woman and lusts for her", but says "whoever looks at a woman to lust for her". There is a big difference in meaning with the one little word change. Looking at someone to lust for them is acting on a thought. Acting on an evil thought is a defilement that comes out of a man.
I don't think we can completely control what kinds of thoughts enter our mind, other than praying to God to keep our thoughts pure. I think the devil plays a big role in putting thoughts into our minds. So when we act on one of these thoughts can we use the excuse that the "devil made me do it"? No, the devil may have put the thought in your head, but if you choose to act on it, it is your doing, and your defilement, and your commitment of sin.
So how do you defend yourself, when you can't help your actions based on the uncontrollable thoughts that the devil may give you. Jesus says you can control them. He says there are measures you can take to make sure you cannot carry out those thoughts. Get rid of or remove yourself from whatever causes the temptation. It may be as simple as staying away from what causes the temptation, or it may be as extreme as making sure there is no way you could ever do such a thing, as Jesus states in bodily mutilation. I don't think He is promoting bodily mutilation. You should be able to find some action to help you that isn't quite as extreme. I think Jesus uses the example of bodily mutilation to emphasize just how important it is that you find a way to control your actions. For it would be better to lose your sight or lose a limb than to allow yourself to be lead into sin. Being blind or lame would be difficult, but would be as nothing compared to being in hell.
Doing whatever you can to prevent yourself from acting on thoughts of sin does not just apply to the sin of adultery, but to all sin. Jesus says to remove whatever is causing you to sin.