I think it is a terrible summary, but I'll assume it's because of a failure to communicate on my part.
(3) does not follow from (2) and (1) together. What does follow is a need to analyse the argument further. I didn't make this analysis, I only sketched it in broad outlines. Let me put out my thoughts in a different way:
A) There is no causal link established between measured sex-linked variation in humans and social organization in humans, beyond the obvious one that the distribution of physical size and strength is strongly bimodal and this can be observed as an underlying basis for many social relationships.
B) And yet, the argument is made. The argument is made and it is presumed to be rational, objective, air-tight, and deductive. In fact, it is an abductive argument: It claims that sex-linked variation is the most likely and simplest basis for the observed social organization that leads to the stratification of society based on binary genders in bourgeois society.
I am specifying bourgeois society and assuming we are talking about modern capitalist societies, because that's the one I live in and the one I have most to gain in analyzing, and also the one whose ideology and real power dominates the planet as a whole.
C) I can accrue a great deal of data that demonstrates corroborated observations that the dynamics of gender in bourgeois society are actively maintained through institutional and systemic structure. This data ranges from the scientific, collected in aggregate, to the personal experiences of a diverse group of individuals.
A huge amount of human effort and ideology goes into gender as a social structure. This expenditure of labor and thought is prescriptive in nature, not descriptive. The argument being made neither explains their existence nor their dynamics because it solely describes variation at a very basic level between human beings. If human beings just naturally organized themselves along the gender lines we see, there would be no need for institutions, social mores, and propaganda to maintain those lines. What we see historically though, is that these lines were drawn by human hands, then dug out, then built upon with concrete, and then stringed up with barbed wire. The argument does not take this historical and sociological context into account and does not even try to exempt our present societies from that context.
D) This situation is incongruous. The argument is not only given more strength than it merits, its conclusions are essentially determined within the philosophical framework it is formulated in, and further it does not account for social and personal observations about gender dynamics. A criticism of the underlying ideology that produces and reproduces this argument is required. I conjecture that the underlying ideology is neoliberal rationality and sketched what that means.
I think danharaj's point was that the effects of sex-linked variance -- regardless of how big they are -- have not been shown to be meaningful compared to the other factors that affect human society. However, it gives the illusion of scientific rigor to use observable sex-linked variances to explain our social condition; in reality there are many possible alternative explanations that are never addressed, and many confounding variables that aren't controlled.
1) Women are suffering under the current power structure.
2) Interpersonal variation is being used as an argument to support the current power structure.
3) Therefore, as a good person I must deny interpersonal variation.