Is expiation required for someone who intended to break their fast while traveling but did not eat anything, then had intercourse with his wife, believing there was no harm in that?

It is permissible for a traveler to break their fast in Ramadan (definition). However, scholars differ on whether it is permissible for them to break the fast on a day they intended to fast. The Hanafis, Malikis, and one view among the Shafi'is hold that breaking the fast is not permissible except for an excuse. In contrast, the Hanbalis and the prevailing view among the Shafi'is permit breaking the fast, citing the Hadith (definition) of Ibn Abbas that the Prophet, peace and blessings be upon him, broke his fast after he had already started fasting during his journey. This latter opinion is the more preponderant. At that point, the traveler may break their fast with anything permissible.