The masters' concept of "loving" is whatever they want it to be. "I'm doing this for your own good," "you need that rebelliousness beat out of you." Very few people ever think of themselves as cruel or abusive, they're just doing what needs to be done.
In the real world and not Marston's fantasies, "loving authority" figures would be corrupted unless they themselves were supervised. I notice he never has a message of young women becoming independent, thinking for themselves, making their own decisions... traits we see as desirable.