expression, noun
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.
彼女は道ならぬ恋に落ちてしまった。
She fell into a forbidden love affair.
道ならぬ恋は、いつか終わりが来るものだ。
An illicit love affair is bound to end someday.
The phrase combines 道 (path, way, moral path) with the classical negative auxiliary ぬ (not) and 恋 (love), literally 'love that is not the proper path'. The exact historical origin is uncertain, but it has been used in literature to describe morally unacceptable love.