Ohr Hachayim (Acharei Mos, 18:2)
Part 1
 
It is known that all the Mitzvos that Hashem commanded his holy nation are Mitzvos that a man can stand by and bring within himself a desire to do them, besides for the Mitzva of staying away from arayos (illicit sexual relations) which is something that a person craves and his desires force him to act, unless one makes an effort to stay far away from two things: (1) Seeing with the eyes and (2) Thinking about it. However, if a person does not keep away from these two aspects (sight and thought), a man will not be able to control himself and rid himself of it (the desire). 

As long as one does not stay away from looking, even if he stays away from thinking about it, he will not be able to control himself and rid himself of the inevitable desire. As we can see from the story of R' Amram - the Rebbe of all Chassidim (Kidushin 81a), that even though he was far from thinking about these things, he was won over by the aspect of "sight" (to see the full story of Rav Amram see Chizuk e-mail #275 on this page). And we can also see this from the story of Reb Masya Ben Charash (Yalkut Shimoni; Veyechi) who chose to blind himself when he felt that he would be forced to come to bad deeds through the sight of his eyes, even though he was surely far from thinking of these things - as is clear in the story (see the story in Chizuk e-mail #46 on this page).

And if one does not distance himself from thinking about these things, his thoughts will also force him to seek out and lust after the deed. As we can see from the story in the Gemara (Avodah Zara 22b) of a non-Jew who bought a thigh of meat in the market place, cut into it and used it to pleasure himself, and then cooked it and ate it. This comes from being steeped in the aspect of "thought" which forced him to do these deeds even without seeing anything. 

So we see that with either of these two aspects, sight or thought, it becomes close to impossible for a person to control himself in this area. And it goes without saying that if one looks AND thinks about these things, he will be very weak when facing this desire. And it also goes without saying, that if one actually begins to taste from the bad, he will be completely given over into the hands of his desires. 

Part 2
 
And the only way that a man can wage war with this powerful desire is by removing from himself these two aspects (improper sights and lustful thoughts). And by doing this, one instills within himself that he should not lust after this, and he will be able to control his natural desires. And this is the reason that Chaza"l say (Brachos 34b) "In the place that Balei Teshuvah stand, even the completely righteous cannot stand". For the natural good will of the completely righteous wins over their desires without great effort, which is not the case with a Ba'al Teshuvah.

And if one should ask, "How can a natural person be in control of his desires which "force" him to act? After all, it does not make sense that Hashem would obligate all men equally to guard themselves in this area, only people who can stand up and have the will power to deny this desire; and these are people who never came into the tests of seeing improper sights and thinking lustful thoughts. Only for such people did the Torah command these Mitzvos. But for people who already fell into these things, it is naturally impossible for a man to control himself and hold back from this craving that forces him to act!"

To counter this claim Hashem wrote in the Torah with pleasant words of life, and the Parsha of Arayos starts out with the words "Speak to the Children of Israel and say to them, I am Hashem your G-d". For it is true that in the human race, besides for the Jewish people, they can claim this complaint and say that they did not find it in their strength to deny the power of this intense desire from themselves. However, with you, the Children of Israel, because I am Hashem your G-d - that is, you can achieve understanding and awareness of G-d - and through this G-dly strength, you will win over the natural physical drives.

Part 3
 
The mind can control the matter. So when a Jewish person accepts upon himself to be "one" with Hashem his G-d (di'veikus), he will be in control of his nature. As Chazal say, "The hearts of the Tzadikim are given over in their hands, but the hearts of Resha'im control them". The meaning of this is, that desire and lust are in the heart of a man, yet his "will power" is in his mind. And these are the two levels that the Master of the World has given man (to work with). And when a person sees something of a sexual nature, his heart will crave it, but this is not the end. The "will power" in his mind can deny himself the desire and he will not act upon it, even though he covets it... And that is the meaning of the words "For I am Hashem your G-d" (in the Parsha of Arayos). This means to say, that since Hashem shines the light of his Shechinah into the soul of a man, through this, one has the strength to win over his desires through his will power...
 
To return to what we were saying... The aspect of thinking about sexual matters strengthens the desire. And one cannot win over this desire unless he distances his mind and thoughts from these things and does not look upon these things. But should his eyes gaze upon them, he is in grave danger, like the story of David Hamelech A"H, that when circumstance brought him to see what he saw, his heart, which had been empty of desire (as the Pasuk says "and my heart is empty inside me") was aroused and he acted. 
 
And even if one just "begins" to think about these things, it will cause a strengthening of his natural desires. And therefore the wisest of all men, Shlomo Hamelech A"H wrote in Mishlei (1); "My son, if the sins will persuade you, do not listen" - meaning, do not even enter into negotiations with your Yetzer Hara, for the longer you think about it, even in a negative light, the more the desire is strengthened and will turn one's will towards it. 
 
And this is also why Hashem says; "Tell them that I am Hashem their G-d", because (those who guard the Bris) are on such a level that Hashem wants to unify his great name upon them. For he who guards his Bris is on a great level. Indeed, there is no greater level than this.