Is what sometimes happens, namely penile erection and seminal emission, as a result of friction between clothing and the penis when looking at something stimulating, considered masturbation? And are there any harms to it, or is there no harm if semen is not emitted?

If the friction is unintentional, scholars differ on whether it constitutes masturbation or not. However, if the friction is intentional, it is prohibited masturbation, because it is a manipulation of the penis to seek pleasure. It remains that looking at what Allah has forbidden is a reprehensible act, and a Muslim must lower his gaze. As for the harms, one can inquire about them from doctors.