Is someone who masturbates without using their hand or fantasies considered sinful, and why, especially since there is no medical research proving its harm except when done excessively?

Masturbation is forbidden in Islamic law, by any means whatsoever. The proof for this is Allah's saying: {And those who guard their private parts Except from their wives or those their right hands possess, for indeed, they are not to be blamed But whoever seeks beyond that, then it is those who are the transgressors}. And the Prophet's (peace and blessings be upon him) saying: "O young men, whoever among you can afford to marry, let him do so, for it helps him lower his gaze and guard his modesty. And whoever is not able to marry, let him fast, for it will be a shield for him [from sexual desire]." If masturbation were permissible, the Prophet (peace and blessings be upon him) would have guided to it instead of fasting. The prohibition of masturbation is based on Islamic legal evidence, not merely on its harm.