I don't mind flat chests, but breasts are not the only physical feature that makes someone look more feminine. It's subtle and I probably can't describe it exactly, but it also has to do with the girth of the torso, the girth of their waist when compared to the torso and hips, arm girth, etc.
It's like how you can look at female powerlifters and somehow you can still see the feminine qualities of their body. Like I said, if the outfits had choices similar to the Bishamo outfit then I think it would make everyone happy. I do realize that it is a lot to ask for two different sets of assets for each outfit, though.