I believe that gender roles are connected to reproductive roles, which are connected to sex. It is reproduction and child rearing that a culture is concerned with, not sexual relations per se. That's why conservative leaders speak of family values, values that engender effective families. There are lots of reasons. A culture, a family, even a churches future depends on the next generation.
More people means more power, more resources, more clout.
A culture, any culture, must be concerned with how many children are born and the stability of the homes these children are born into, because this defines the culture in many ways. For me, looking at gender rules in the context of this reproduction and rearing of children seems to make sense. Girls are sexually desirable, mothers are stable, and so on. There is of course overlap in these roles, but they show the woman going though the process of being a responsible and valuable person who reproduces in culture. The Roman Catholic church holds Mary up as the ideal woman, a mother who played her role in bringing new life into the world.