“The general secular morality standard would be ‘do no harm’.”
I’d take issue with this reasoning. This is not the general secular morality standard, it’s nothing more than Alistair Crowley’s “Do What Thou Wilt” philosophy restated. Most people don’t agree that this is the only standard, because we don’t accept that things which cause harm to one’s self are morally acceptable.
I’m not sure secularists would say that masturbation is harmful. If you don’t believe it is a sin that will damn you for all eternity, there is no harm caused as far as you are concerned...