Actually, he does have that message fairly frequently.
Whether the masters would say they were being loving or not is irrelevant, they wouldn't actually BE loving. That's the difference. However they rationalized it, they still would be acting in their own interest and not that of the slaves. Marston makes that distinction, he doesn't simply say that all submission is good. In his version of things, the masters exist to make the slaves happy as much or more than the slaves exist to make the masters happy, so I really doubt that he would apply his theories about "loving submission" to the civil war. I suspect he'd be horrified if anyone did so and, if they did so in a Woody Allen movie, he'd tell them "you know nothing about my work."