There certainly is a difference between a woman wearing a wide array of styles (including anything available for men whether cut for a woman or not) and a woman intentionally trying to portray a man. I think there are a good number of the latter, though not as many as the other way around. The issue of women having more available to them is a huge factor, but not because it eliminates the need so much as it removes the possibility of the powerful identification a man can feel when wearing items that are "off limits" except to the opposite gender (like panties, heels, hose, dress, etc.). There is simply no good comparison that could generate the same psychic senstation for a woman. However, I think the more important issue is the rigidity and narrowness of the gender role for men in western culture when compared to that of women. This makes men all the more predisposed to wanted to have such a "crossing" sensation in the first place (like crossing a bridge out of bondage). As for body form, I think it is mostly a non-issue because, whereas breasts compose a profound element of the look of a woman, a penis does not do the same for a man (it is largely hidden in our clothes). Similarly, whereas a penis destroys a female look (hence tucking), breasts generally would not do the same to a male look. The only real issue for a male look might be facial hair, but most men don't keep theirs and, practically speaking, in the end only a small percentage of FTM cders would end up pursuing this anyway, making their numbers that much smaller.