If the one who had intercourse forgetfully withdraws immediately upon remembering that he is fasting, then his fast is not invalidated, even if he ejaculates after withdrawing; because it is like wet dreams. However, if he remains without withdrawing and does not move, he must make up the fast. Refer to Rad al-Muhtar 2: 99, Al-Hadiyah al-Ala'iyah p162, and Al-Mabsut 3: 141.