Male disposability is the notion that men and boys should surrender their life for the greater good of the community and in particular to facilitate the survival of women and girls.
Male disposability has a basis in evolutionary theory. When the females of a species invest significantly more energy in to gestation than males, as is the case for humans, then the size of the next generation is largely constrained by the number of fertile females. As a result of this a species can generally tolerate the loss of males more easily than the loss of females. The loss of females will constrain the size of the next generation, the loss of males will constrain genetic diversity.
We humans are now a special case. The Earth carries more than seven billion people today and could be carrying 10 or 11 billion within a few decades. We no longer need the ability to rapidly recover numbers, thus any presumed bias towards male disposability that existed in the past no longer needs to apply. We've won the survival race.
Male disposability is an artifact of the past and that's where it should stay.