Engels seems to argue that there once was a "sexual" division of labor that "naturally" arose but was not hierarchical nonetheless, at least under the conditions of a matrilineal, primitive society without private property (so does Marx, except that Marx's ideas on this topic are not elaborated in detail). I once thought that Marx & Engels were altogether incorrect in thinking so, but since then I've changed my mind a little bit, and my provisional thoughts on the matter are as follows.
There are many questions to be asked. To what extent did biological reproduction dominate the lives of women in the primitive society without private property? To what extent were they able to have control over their own bodies in the sphere of procreation? How often did they have sex? How often did they conceive (the rate of conception determined by health, not just by the frequency of intercourse)? At what age did women become fertile (in a primitive society it is likely that the onset of menses was much later than now, due to [by our standards] comparatively austere diet, need for physical exertion to produce the necessities of life, etc.)? At what age did they experience menopause? How long did they live, and what was the average proportion of fertile years in their life spans? [Allow me to insert a "footnote" here."]
***** Ruth Hubbard, "Rethinking Women's Biology," _The Politics of Women's Biology_, pp. 125-6
Rose Frisch (1988) has shown that, during the last century, the age of onset of menstruation has gone down and the age of cessation has gone up in both Europe and the United States. From this change and from studies of the menstrual patterns of athletes, she has concluded that nutrition and exercise strongly affect these parameters, probably by influencing the amounts of stored body fat. She suggests that women need to accumulate a threshold amount of fat in order to establish the hormonal cycles that regulate menstruation....
...Anthropologists observing the !Kung, a group of foragers living in the Kalahari desert in southern Africa, have noted that their menstrual and reproductive histories are quite different from what we in the West think of as "normal".... !Kung women and men collect their food, and as is true in most foraging societies, women provide most of it which involves a good deal of walking and carrying. The !Kung diet is plentiful and nutritionally adequate but very different from ours because it is relatively high in complex carbohydrates and plant proteins but low in animal proteins and fats.
Presumably because of their high activity levels and their diet young !Kung women do not begin to menstruate until they are about eighteen years old, by which time they already tend to be...sexually active. Like girls in the West, they tend not to ovulate during their first few cycles. They therefore experience their first pregnancy when they are about nineteen and have a first child at perhaps twenty. They nurse that child for two or three years....[D]ue to the frequent nursing, !Kung women tend not to menstruate or ovulate for almost the entire time they suckle their children....
...[!Kung] women have a shorter reproductive span, during which they tend to bear no more than four or five children.... *****
In addition, there are more clearly social and technological questions to be asked, humans being knowledge- & tool-producing animals from the beginning of history. Were they able to space their pregnancies? Did they have say in when to have sex? Did they practice birth control (e.g. coitus interruptus, vaginal suppositories, etc. -- for more on birth control in pre-industrial society, see Linda Gordon's "The Folklore of Birth Control," _Women's Body, Women's Right_)? Did they know how to abort? Did they often or seldom practice infanticide (by exposure & non-feeding)? Did they raise children communally?
If there was a society that had shorter reproductive spans (than the present), practices of birth control (from coitus interruptus to infanticide), & communal child-rearing, we may speculate there was no ground for a division of labor that would produce gender. On the other hand, if one or more of the above was missing in a given society, as it was more likely, women in such a society must have been dominated by reproduction to a much greater extent, and then we may speculate that a division of labor must have arisen, due to the lack of social conditions that would enable control over reproduction. Marx & Engels must have thought the latter was the case -- hence their belief in the "sexual" division of labor, since reproduction would be _a_ factor in the division, though it still would not be correct to think of it as a "biological" cause of it, for reproduction was never separate from social practice even in the most primitive of human societies.
Also keep in mind that for Marx, the word "natural" didn't necessarily mean "biologically determined." "Natural" was, for him, often synonymous with "not consciously & rationally controlled."
***** Marx, _The German Ideology_
as long as a cleavage exists between the particular and the common interest, as long, therefore, as activity is *not voluntarily, but naturally divided*, man's own deed becomes an alien power opposed to him, which enslaves him instead of being controlled by him...this fixation of social activity, this consolidation of what we ourselves produce into a material power above us...is one of the chief factors in historical development up till now. *****
The lack of control over reproduction makes women subject to a "natural" division, which is always already socially produced, in that reproduction no more takes place outside of social relations than production does.
In this sense alone, one may use the "sexual" division of labor synonymously with the "gendered" division of labor, in that under the conditions where women lack control over reproduction, the "sexual" becomes gendered. Understood in this fashion, however, the "sexual" doesn't mean biologically determined.
Yoshie