"Surely she lurks as a robber, and increases the faithless among men"
Solomon is trying to teach us what adultery does to those men and women who get seduced into it. It causes those who are faithful to become what they would not become under any other influence other than adultery.
This is the Hebrew word hetep which means prey or robber. An adulteress is like a predator that leaps onto her victim and destroys them. She does not offer them something they need; she baits them into the trap that will change their life forever. She is like a tiger that leaps out of a bush and consumes the victim.
This is the Hebrew word bagad which means treacherously, betrayed, faithless. The idea is that adultery changes a man or a woman and moves them to become treacherous. They will lie and deceive and move across the line of morality to keep receiving the adulterous pleasure.
What very few people realize is how much adultery will change a person. A few moments of sexual pleasure permanently changes and twists one's life. Do not forget the consequences. This is what Solomon wants to drum into our heads.