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 Chaza"l 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 with his will power...
So 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 his 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 the 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.