All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
Whoever forgetfully eats, drinks or sleeps with wife should complete his fast and there is no sin on his part or expiation, because he was forgetful and the Prophet (PBUH) said: "Whoever forgetfully eats or drinks, let him complete his fasting, for Allah Has Provided him with food and drink.” [Al-Bukhari and Muslim]. Moreover, other matters that invalidate fast are judged based on this sound analogy. Al-Imam An-Nawawi said: "If he forgetfully had sex with his wife then his fast isn`t broken, as stated by the scholars of the Shafie school of thought." {Rawdat At-Talibeen, vol.1/pp.272}. In conclusion, your (asker) fast is valid and there is no sin on your part or expiation. And Allah Knows Best.