The closest I can think of is the very powerful scene late in Midnight Express, when Billy's girlfriend visits him in prison (he's been unfairly incarcerated for a minor drug offence by a corrupt legal system determined to make an example). After years in brutal Turkish incarceration surrounded by (mostly) brutal men, the sight of feminine beauty is too much for him. Though they are separated by glass, she takes off her top and he masturbates, whilst she weeps "I wish I could make it better for you". Perhaps not "positive" exactly, but the only remotely moving masturbation scene in cinema history that I can think of.