Why does it say "Shuchah Amukah Zonah"?
R. Yonah: One who approaches the doorway of a Zonah, he is trapped in her, just like one falls in a pit if he does not guard from it.
Malbim: Corresponding to "v'Eineicha Derachai Titzorenah" (26), it says that a Zonah, the Yetzer that desires, is a deep pit. One who is trapped in it, it is hard to leave it.
What is "Tzarah"?
Rashi: It is constricted.
Who is "Nochriyah"?
R. Yonah: Any woman other than his wife. Even if she is not a Zonah, she is like a narrow pit. If he will regularly talk with and look at her face, every day his heart will increase to be drawn after her. It will arouse new desire and a spirit of Zenus, until he will stumble.
What is the comparison to a narrow well?
R. Yonah: Just like well water flows, and whenever he draws from it, new water comes, so from talk [with the Nochriyah] he will come to seeing, and from it to Hirhur, and from it to touching, until he will transgress and fall in the pit of sin.
Malbim: Corresponding to "Tenah Vni Libecha Li" (26), it says that a Nochriyah (heresy) is like a narrow well. One who falls there drowns immediately in evil water, and dies. So one who sinks in heresy, immediately his Nefesh dies, and he will not return to see the light of life.