The simplest assumption is that femboys are underdeveloped futunari. Both types have female figures and male genitalia. During the lecture, they say that femboys are more likely to be born in W-F pairs, in W-M pairs both parents must have a futunari in their ancestors. A recessive gene, scientifically speaking.
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But we can assume that in the world of the game, ordinary boys are transformed (by social and medical methods) into femboys! Perhaps there are not enough women for all men and futunari, so they take measures.
The plot of the game itself speaks in favor of this version. Obviously, at birth, femboys are defined as boys, and they are also brought up as boys. Before reaching puberty age, when (self) occursthe definition of femboy and its adaptation to a new role follows. Moreover, the criteria for determining are strange - body proportions and penis size. It would be more logical to divide by the ability to fertilize. It is even more logical to determine gender at birth or in early childhood, so as not to create problems in the future.
When defining a femboy, his image changes dramatically, from male to female. What is included in the adaptation program. The adaptation program also includes sexual training. All this suggests the unnaturalness of femboys. Otherwise, why forcibly change their image and train them in sex? Nothing is said about similar actions for girls.