Why isn’t Narcissa a Death Eater?

I can only think of two female Death Eaters: Bellatrix Lestrange and Alecto Carrow (the “lumpy” Death Eater’s sister at the end of HBP). Why so few? Narcissa Malfoy’s family, for example, is deeply involved in the Death Eaters, yet she is not. Why not? Why are there so few female Death Eaters?

This might be related to the description of the Slytherin Quidditch team as always being all male. Perhaps the Slytherin/Death Eater crowd is supposed to be patriarchal, with the women staying behind the scenes while the men go out and do the dirty work.