I wonder if it is a sin for a Muslim to have sexual thoughts which makes one ejaculate. I have asked many brothers but never got a satisfactory answer with proof.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Thinking deliberately about women to get pleasure by ejaculation is not permissible. This act is considered as masturbation or even more offensive since the person who does so visualizes the pictures of women in his mind. This act becomes more repulsive if one thinks about a particular woman to get ejaculation.

Therefore, this practice is considered among the minor Zina acts mentioned in the following Hadeeth:

Abu Hurayrah narrated that the Prophet said: “Allah has decreed for every son of Aadam his share of Zina which he will inevitably commit. The Zina of the eyes is (unlawful) looking, the Zina of the ears is (unlawful) listening, the Zina of the tongue is (unlawful) speech, the Zina of the hand is (unlawful) gripping, the Zina of the leg is walking (to places where a person intends to commit immorality), the heart yearns and desires, and the (person through his) private parts may or may not put that (desire) to effect.” [Al-Bukhari and Muslim]

Allah knows best.