I could see that the men come with non-functional back sacks, or empty cavities where the sack should have been, like how men have nonfunctioning nipples... But why differentiate genders if you're not going to oblige to keep the gender understandable.
It's not like Janeway would hook up with an perceptible gender in Workforce and never explan how she interlocks with that persons body with her own womanly bits..