Jammer, forgive me, but you come across like, well, a social engineer. It seems (well, no, now you've made it explicit) the very concept of separate-sex classes is inherently evil, like Jim Crow. You're going to change the culture, make it better, until co-ed classes are the norm. Why? Why do you care? This is not an ethics question at all.
There's nothing unethical in realizing that simply jumbling the sexes together hurly-burly is not necessarily constructive, quite especially in this field. And the racial comparison is woefully misplaced.
Unethical is teaching your students to different standards, with the obvious exception of recognizing the different aptitudes, experience and interests of your students (as our music teacher explained). Unethical is using one class primarily as a vehicle for recruiting for your next class.
Keep your standards. Good on you. And keep separate classes. There is nothing mercenary in giving your clients what they want.
Really, relax, friend. Men and women are quite different, and their needs and experience (hell, their physiology and neurology) are different.