Does a man have to perform ghusl if semen is discharged merely by looking at his wife?

You are not asked not to look at what Allah has made permissible, but if looking results in the emission of semen, this is not incontinence. Rather, you must perform ghusl (ritual bath), because incontinence is restricted to what cannot be removed, and you can control your gaze. So, if you look and semen is emitted, then what is incumbent upon someone in a state of janabah (major ritual impurity) becomes incumbent upon you.