"You can come to Atlanta and attend classes at various studios for a while. You'll quickly see that men outnumber women at these classes 80% of the time."
Pro/am siphoning off the ladies is no doubt part of the problem, but it may not be the whole story.
Many group class formats are more useful or at least more tolerable for leaders than for followers.
Much of what can be easily presented in a group class concerns the men, and regardless if they are or are not picking it up well, there is at least something for them to do. In contrast, the skills missing in most intermediate ladies are quite a bit harder to address in a group class setting - they have nothing to do with choreography, but are much more about details of movement technique and philosophy of partnering. I've seen that effectively addressed in a group class, but it's unusual.