1)
What does the verse teach about an adulteress?
1.
Rashi: You are the adulteress. Radak ? you are under Hashem, and you took (served) other gods. Our verse explains the Mashal; Bi'ah is a metaphor for idolatry.
2)
What do we learn from "Tachas Ishah"?
1.
Rashi: While she is under her husband (married), she puts her eyes on others.
2.
Malbim: She is not a Zonah Hefker to openly leave her husband's Reshus, only she does adultery for desire. She takes foreign men while under her husband's Reshus, covertly.