How can a handshake between a man and a woman be an مقدمة (introduction) to zina (unlawful sexual intercourse)?

It is from Allah's mercy that He forbids whatever leads to the forbidden, knowing the nature of the human soul. So how can He forbid zina (unlawful sexual intercourse) yet permit what leads to it, such as shaking hands, looking, and seclusion (with a non-mahram (definition))? This is like the saying: "He threw him into the sea, bound, and then told him: 'Beware, beware of getting wet by the water.'" The Sharia (definition) of Islam, not surprisingly, aligns with intellects and innate dispositions. Forbidding what leads to the forbidden is a Sharia principle, as means take the rulings of their ends.