Foundation of the Building
Sex and love are the deepest human emotions and fixing these emotions fixes a person deeper than anything else. That's why in Kabbalah, the Seffirah of "Yesod" refers to the Brit. "Yesod" means "foundation". Guarding the Brit is the foundation of Judaism.
The positive commandments of the Torah are like a building. The negative commandments, which are what you don't do, what's underground, what people don't see - that's the foundation of the building. If the foundation is weak, then the whole building is in danger of falling. No matter how much Torah and Mitzvos a person has, if he is not careful in guarding the "Yesod", he can lose it all in one moment. That's why people who are pogem the bris feel so precarious with their Yiddishkeit, they feel they are constantly walking on a tightrope above the abyss. If the foundation of the building is strong though, then the building will last, no matter what it looks like above ground!