Is he who ejaculates forgetfully after fondling his wife during a Ramadan day, while remembering that he is fasting, sinful?

If you fondled your wife to the point of semen discharge while forgetful, your fast is valid, and there is no make-up [fast] upon you, nor sin, because you are in the position of one who ate or drank forgetfully. The correct view is that knowledge is a condition, based on the indications of the Qur’an and Sunnah. Thus, the conditions for things that break the fast are three: knowledge, remembrance, and intention.