Q: Is Qada necessary if a person ejaculates without manual stimulation because of viewing explicit material while fasting?
A: Whilst fasting, just as it is impermissible to indulge in sexual activities similarly it is impermissible to fantasize about sexual activities. As far as looking at pornography is concerned it is totally prohibited irrespective of whether one is fasting or not. Committing these major sins in the state of fasting destroys the reward of the fast completely.
However, if a person ejaculates due to fantasizing or looking at pornography without anything touching or rubbing his private part the fast will not break.
If one masturbates by touching or rubbing his private part on something whilst fasting the fast will be nullified. Only Qadha will be Wajib.
(Durrul Mukhtar 2:396, Al Fatawa Al Hindiyyah 1:204, Maraaqil Falaah pg 658)