"Human character," not "human nature." Human charcter would be "he's an evil person, for whatever reason, but that can be changed." Human nature would be "people are intrinsically evil, no matter what." Take out the intermediary step of being tied up by a beautiful woman, and all Marston is saying is that love and wisdom can turn evil into good. Of course, I don't think anyone wants to take out that intermediary step.
It probably says something about the world in general that when this was published most readers focused on the love and wisdom part and didn't pay attention to the bondage, but now most readers focus on the bondage and overlook the need for love and wisdom to be behind it.