First, it violates God’s holy standard (see Ex 20:14). Second, its consequences are devastating. Proverbs focuses on the consequences, stressing the practical reasons that adultery should be avoided at all costs.
The writer acknowledges the powerful and tantalizing appeal of adultery. Adultery pretends to offer pleasure and sexual release with few risks. The secrecy, intrigue and forbidden nature of the act only seem to add to its relentless pull.
Yet appearances are deceiving. What begins as sweet desire quickly turns bitter. Adultery destroys reputations, character and marriages; the permanent costs immeasurably outweigh the momentary rewards.
Even adultery done and kept in secret is fully exposed to a God who judges righteously.