There's a fundamental and elementary piece from the holy Ohr Hachayim that gives a clear perspective on the powerful nature of carnal desire, and helps us understand how Hashem expects us to remain holy in spite of it being against human nature. In honor of Shovavim, I would like to bring this Ohr Hachayim now in a few stages. To see the original text in Hebrew, click here (the important parts are marked in yellow, and the VERY important parts in red).
It is known that all the Mitzvos that Hashem commanded his holy nation are Mitzvos that a man can stand by and cause himself a desire to do, besides for the Mitzva of staying away from arayos (illicit sexual relations) which is something that a person craves and his desires forces him to act, unless one makes an effort to stay far away from two things: 1) Seeing with the eyes, 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.
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 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, 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.