Chazal say that every human has 248 limbs that correspond to the 248 Mitzvos Aseh, and 365 sinews that correspond to the 365 Mitzvos Lo Saseh. Altogether = 613 Mitzvos. The Sefarim bring down that the Bris (or Yesod) is the one limb in a person that reflects all the others. Why? Because through the Yesod, all the other limbs of a person are created. The Bris contains within it the power to create all the other limbs. Therefore, the Sefarim say, the main test of a man in this world is in relation to the bris, and when one successfully guards it, it is as if he guarded his entire body as well!
Rebbe Nachman says, that these desires are the greatest test that a man faces in this world (Rabbi Nachman's Wisdom 115). He said that he was able to overcome this desire, but he had to cry out to Hashem again and again. The Satan wanted so much for him to slip on this desire that he was willing to let Rebbe Nachman overcome every other desire, if only he would slip up on this once. Rebbe Nachman did the opposite though, and said he would ignore his other desires and not work to control them at all, but the sexual desire he would completely eradicate (Shevachay Haran 16). As a young man, even amidst the fire burning inside him, he overcame this desire completely. He later said, "I do not have any feeling of desire at all. Men and women are all the same to me". When he came in contact with a woman, he didn't even have an extraneous thought. (ibid)
Our sages called Shmiras Habris "Yesod", meaning "Foundation". The foundation of a building is "underground" and no one sees it, yet it holds up the entire building! Shmiras Habris is the hidden part of a Jew, it's the real you. If the foundation of a Jew is weak, his whole spiritual structure is fragile and in grave danger of collapse. And if the "foundation" is strong, one can build sky-scrapers of holiness on top of it!