illicit love affair; forbidden love
A set phrase for a romantic relationship that goes against social norms or moral expectations, such as adultery or a love that cannot be openly accepted.
She fell into a forbidden love affair.
An illicit love affair is bound to end someday.
From 道, “proper or moral way,” and ならぬ, the literary negative form of なる, followed by 恋: literally, “love that is not in accordance with the proper way.”