Is it permissible for sexual intercourse to occur forgetfully during the daytime in Ramadan and consequently incur expiation, knowing that eating and drinking forgetfully does not necessitate expiation?

A Muslim must guard their tongue. Regarding your question, expiation (kaffarah) is incumbent upon one who intentionally, knowingly, and while fasting, has intercourse with his wife during the daytime in Ramadan. However, one who has intercourse forgetfully is not liable for expiation; rather, he must make up that day, for the Prophet, peace and blessings of Allah be upon him, said: "Indeed, Allah has pardoned for my nation their mistakes, their forgetfulness, and what they are coerced into." A person can have intercourse while being forgetful or heedless of his fast.